The body function and external morphology of animals with long life span are affected by the environment. Changes in environmental factors, such as temperature, humidity, and illumination, affect the animal's appearance and its internal functions through changes in physiological mechanisms. Many species of mammals are characterized by a layer of hair on the skin. Hair, being constantly exposed to the outside environment, directly indicates functional adaptations to the environment. Adaptation to the environment is caused by a long-lasting selection pressure on a trait as a result of natural selection. To fit the environment, animal can change a series of traits, such as color and body shape. The hairs, which serve for thermal insulation and protection, are the result of a long adaptive evolution. As a result of geographic isolation, populations of one species that are isolated from each other for a long time undergo speciation, resulting in subspecies or new species. Morphological structure of hairs covering different body regions is the result of different functional adaptations. Often there is a gradient in hair variation from the body to the end of the limbs in many mammals. Siberian weasel is a valuable fur animal and a subject of various studies in China. The animal molts seasonally starting from tail to hips, back, and neck, and lastly head and limbs in the fall, and in the opposite direction in the spring. The hair type in Siberian weasel is complex. On the basis of its shape and structure, hair is divided into guard hair, sub-awn, vellus awn, undercoat hair, and bristles. In this study, we measured the length and diameter of guard hair, sub-awn, vellus awn, and undercoat hair, all of which were sampled from the center of the back and venter, the lower end of hind limb, and bristles from the upper side of the hind toe. These hairs were collected from winter pelage of 20 adult Siberian weasels located in Tonghe forest area of Heilongjiang Province. Results showed that the length and diameter of the four types of hair always decreased at the same position. We detected significant correlation between the length of the four hair types (P< 0.01) and significant positive correlation between the diameter of guard hairs and sub-awns (P< 0.01) as well as between vellus awns and undercoat hairs (P< 0.01). The character of each hair type provided the potential to properly insulate and protect animal body as a whole. Thus, the insulation provided by the hair decreased from the back to the hind toe, whereas the protection increased. The hair traits differed among body regions in relation to different temperatures at those parts. We collected and analyzed morphological characters of the guard hair including its length and width, cuticular scale pattern, and medulla. We found significant difference in hair density between the winter and summer coat, hair length, and proportion of absent medulla in guard hair, and we discuss the adaptive mechanism of this variation. Pelage is found only in mammals, where during the course of evolution, its primary role became insulation and protection from the elements. The morphology of the pelage related to the insulation and protection function differed significantly between body regions, resulting from the differentiation of the characteristics during the course of adaptation of pelage. There is a visible gradient in its morphology from the trunk to the limbs. This study reported functional adaptability of the morphological structure of winter pelage of Siberian weasel. The fur was sampled from 20 Siberian weasels (10 males and 10 females) from Longkou Forest Farm of Tonghe in Xiaoxingan Mountain, Heilongjiang Province, in winter from December 2005 to February 2006. Five hairs of each of the four hair types (unbent awn, sub-awn, vellus awn, undercoat hair) were sampled from different parts of the body including dorsal back, venter, and lower hind leg, and five bristles were sampled from upper side of hind claw. The length of the hair, medulla, and non-medulla tip as well as the diameter of the hair and medulla were measured in all sampled hairs and the measurements were used to calculate the proportion of the length of medulla, non-medulla tip, and indexes of medulla (proportion of medulla diameter to hair diameter). All the measurements were carried out by using microscope photography and its supporting software for photo processing and measuring. The analyses indicated a significant correlation between the length of the four hair types and a significant positive correlation between the diameter of unbent awn and sub-awn (P< 0.01) and between vellus awn and undercoat hair (P< 0.01); the hair length played a very important role in the insulation and protection properties of the pelage. Furthermore, the morphological properties of the Siberian weasel winter pelage showed a gradual change from the trunk to the limbs, indicating that the gradient in the insulation and protection function of the pelage corresponds to the heterothermy. Observed polarization of winter pelage in Siberian weasel is the result of adaptation to cold environment in winter. This study provided insights into the strategies of Siberian weasel to adapt to cold climate in the Tonghe forest area.