Abstract: From 1973-1980, a total of 3,571 house mice were captured from various habitats and of all seasons in Taxihe, Manas county, Xinjiang Uygur Autonomous Region. Measurements of length and weight of body were taken for studying the relative fatness of the mice.The auther calculated it by means of the following formula which was introduced into small mammal's investigation by Hsia et al (1963) from ichthological study:K= 100W/L³,where K represents the relative fatness, W the weight in gram, L the length of head and body in cm.The results show that the relative fatness of house mice has the following features :There are no obvious differences in sex or in various groups of age (Table1 ) .The difference of relative fatness between various habitats is evident especially in October (Table 2 ) .There are obvious seasonal fluctuations. It is medium in spring (K = 3.19), the lowest in summer (K = 3.11 ), the highest in the middle of october (K = 3.38 ), but before winter coming, it drops evidently and keeps the valus until next spring (K=3.14 ) (Table 3, Fig. 1 ) .