Geographic variation in mean body size along latitudinal and altitudinal gradients is a commonplace occurrence, but little is known about the relative importance of different factors underlying this clinal variation in ectothermic animals. We investigated altitudinal variation in mean body size, age and growth rate in Nanorana parkeri, a ranid exhibiting the highest altitudinal distribution range on the Tibetan plateau. Data collected from six populations covering an altitudinal span of 3800–4700 m showed that the mean body size declined with increasing altitude, and hence, the data conformed to converse Bergmann's rule. Frogs from high-altitude populations exhibited delayed metamorphosis and sexual maturity and had greater average age but lower growth rate than frogs from low-altitude populations. Thus, the proximate reason for the altitudinal decline in mean body size – despite of increased average age towards high altitudes – is likely be the reduction of growth rate at high-altitude localities. We discuss the possibility that physiological advantages provided by small body and egg size could be adaptive in the extreme high-altitude environments characterized by low oxygen supply.