Abstract: The tail ratios of 159 mice from 9 regions in China were measured. It is clear that the tail ratios of mice are closely correlated with the average air temperature in a year in their living region (r=0.96 y=1.45x+63,75). There are significant differences in the tail ratios among mice from cold, warm and hot region (P＜0.01). The mechanism of tail length variation and the causes of the complicity of subspecies classification of mice were discussed.