Pelage is one of the most important characteristics of mammals. The research about the association between molting and environment is an important aspect in the study of animal ecology. Mammals' autumn molting is completely to taking off their summer hairs and growing rich hairs in winter. But the quantitative studies of thermal insulation of animals' pelage which was changing in the autumn molting process have not been involved. Our objective was to find out the relationship between characteristics and heat insulation of hair in squirrel,Sciurus vulgaris manchuricus,that is molting in autumn. In order to study the association between hair length and pelage thermal insulation property, we measured hair length and tested thermal insulation of skins of 27 female squirrels (Sciurus vulgaris manchuricus) (twenty-five adults and two subadults) during autumn molting period. The hairs (5 intermediate guard hairs and 5 undercoat hairs) were sampled from the mid back of each skin and thermal insulation was tested at the same site. Results showed that 1) with the progression of autumn molting, the pelage thermal conductivity decreased in association with increment of winter hair length, and reached stable at the end of hair growth. 2) Significant positive associations were detected between pairs of four hair indexes namely the length of intermediate guard hairs, length of under hairs, root of medulla-absent proportion in intermediate guard hairs and under hairs (P<0.01). Meanwhile, these four indexes were also significantly negatively associated with thermal conductivity coefficient (P<0.01). These results suggest pelage thermal conductivity is effected by the thickness of under fur. In conclusion, the results reflect the specific changes and interrelation of hair length, the extent of growth and thermal insulation in the every time stage during autumn molting period.