Relative fatness index is widely used to uncover the relationship between environment factors and animal individual's survival and reproduction. Changes in relative fatness of Apodemus draco and Niviventer confucianus across age classes and seasons were investigated from April to December 2008 in Fengtongzhai Nature Reserve, Sichuan province, China. The results showed that differences of the relative fatness between males and females for A.draco across age groups were not significant. However, it was significantly different among seasons for their relative fatness, and changed from the highest in olds, higher in adults, lower in sub-adults and then the lowest in juveniles. The relative fatness in sub-adults was not significantly different among seasons, contrary for adults which was highest in spring while lower in summer-autumn and winter. For N.confucianus, its fatness was not significantly different across age groups. However, for N.confucianus sub-adults, their relative fatness was significantly different among seasons. The relative fatness for the two species was not significantly correlated with elevations. Higher relative fatness of A.draco in old classes than those in juvenile classes probably implied that the old individual possessed high ability to adapted to changing environment conditions. Meanwhile, the relative fatness of seasonal changes in adults may be affected by availability of food resources and breeding energy demand across seasons. Adult and old individuals of N.confucianus can maintain stable relative fatness through some measures. However, regulating ability of N.confucianus sub-adult ones is relative weaker, which results in the lowest fatness in summer-autumn.