The mammalian age determination is one of the basic aspects in the study of animal population ecology, while the method of rodent age determination has great significance in the prediction and control of rodents in practice. Although ecology, disease, evolution of Northern Three-toed Jerboa (Dipus sagitta) have been investigated in great detail, few studies have focused on its age determination.125 samples of Dipus sagitta (43 females and 82 males) in Hunshandake Sandy Land in Inner Mongolia from 2002 to 2003, were used to make age estimation according to carcass weight as a major indicator. We recorded Dipus sagitta samples of each capture the body length, body weight, carcass weight, propagation characteristics. The data on male and female individual carcass weight were analyzed by using t-test. Results show that the difference of male and female Dipus sagitta weight is not obvious (P>0.05) (Table 1). Using body weight without viscera as the standard, according to characteristics of the frequency distribution and the appearing time (month) (Figure 1), the Dipus sagitta can be divided into 3 natural age groups: group I (0-1 years old): body weight without viscera≤71g; group II (1-2 years old): 7190g. By inter-group differences t-test, adjacent difference between the two groups was significant (P<0.01)(Table 2). And as population age growth, the carcass weight increased, indicating that this division standard for the identification of Dipus sagitta age, with statistical significance. According to carcass weight as the division standard for each age group body weight, the data of body length are shown in table 3. Dipus sagitta in group I and II have a rapid growth in body length and body weight, while in group II and III, the body length growth rate decreased obviously. The method of identification Dipus sagitta age by carcass weight is relatively simple and accurate.