The present study investigated altitudinal variation in sexual size dimorphism of a Tibetan frog Nanorana parkeri. Size dimorphism was female-biased in all populations, although this bias became less at higher altitudes because of a steeper altitudinal decrease in female size than male size. Operational sex ratios, an indicator of the opportunity for sexual selection on larger males, changed independently of altitude. Clutch volume, an indicator of the strength of fecundity selection on larger females, was positively with female size, and tended to decrease approaching high altitudes. Females lived longer and grew more slowly than males, and the mean age in both sexes increased and growth rate decreased altitudinally, although the changes were more rapid in females than males. These results suggest that, relative to males, females (i.e. the sex that typically bears greater reproductive costs and experiences stronger directional selection for larger size to take fecundity advantages) should be more sensitive to environments, attaining a larger size via enhancing growth under favourable lower-latitude conditions but a smaller size as a result of retarding growth when conditions become harsher at higher altitudes. This supports the condition-dependence hypothesis with respect to intraspecific variation in sexual size dimorphism. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 107, 558–565.