Although the astragalus(=ankle bone) of mammals is useful for the fossil studies, its size variation and relationship with the body mass have not been well documented. In this study, I examined the relationship between the astragalar size and body mass in land mammals, making regression functions to estimate the body mass from the astragalar size. Using the regression functions, I estimated the body masses of several fossil mammals and made some taxonomic reappraisals. Furthermore, I investigated the intraspecific variation of the astragalus in Japanese Monkey(Macaca fuscata). The correlation between the astragalar size and the molar size was low. There is a difference between the sexes in the astragalar size.