Our objective was to describe thermal insulation of the Siberian Weasel Mustela sibirica manchurica form the Tong River forest of Heilongjiang Province. We collected winter pelages from five male pelages and four female Siberian Weasels and studied the structure of the various layers of fur with respect to thermal insulation capability. The structure and function of each layer differed. From the surface to the inner layer, we identified four layers of thickness (12.7±3.0) mm, (6.0±1.8) mm, (5.5±2.2) mm and (1.4±0.5) mm. The outer layer was palm yellow or golden yellow, consisting of two types of guard hairs of greatest diameter and serving an anti-friction function to protect the lower layers. The second layer was light yellow with four types of hair of small diameter. Two types of lower hair layer were curved, reducing the space between hairs and serving to inhibit air movement. The third layer was gray or grayish white, with four types of hair, all of smaller diameters. The two types of shorter hair in this layer were also curved, further reducing air movement and enhancing thermal capability. The fourth layer was white with short, straight hairs of four types of hair facilitating heat transfer. Thermal insulation indices by layer (outer to inner) were 16.11%, 27.40%, 44.40% and 12.09%. Compared to abdominal fur, the dorsal fur was darker and thicker, with stronger thermal insulation. Fur of male weasels was thicker than in females, and the thermal insulation capacity in males was greater. We conclude that each layer of Siberian Weasel pelage plays a role distinct from other layers and this is an adaptation to enhance survival in the cold Siberian environment.