Identifying the patterns of body size and age in relation to altitudes may help us to understand the evolution of life history of organisms. With this view, we compared the demographic traits of a subtropical frog, Rana limnocharis, between populations from southwestern China located at two altitudes (310 and 800 m) with distinct mean annual temperature (17.1°C and 15.2°C). Overall, frogs from the higher altitude tended to be significantly larger in body size. Within each population, adult females were markedly larger and older than males. However, average age between high altitude and low altitude was not strongly different within each sex. There were significant relationships between age and body size within each sex in both altitudes. ANCOVAs revealed that body size within sexes differed significantly between altitudes when the effect of age was removed. Our findings suggest that factors other than age may contribute to differences in body size between altitudes, such as air temperature and breeding season length.