1: The decrease of temperatures along an elevation gradient imposes physiological constraints on reptiles that ultimately determine their distribution ranges. Forest patterns are likely to interact with this process, but very few studies have examined their contribution in determining distribution limits. 2: We examined the role played by thermal physiology and forest cover in determining the elevational ranges of a lizard, Eutropis longicaudata. We integrated this species' thermal traits in simulating its maximum activity time under different conditions of forest cover and elevation using a NicheMapR model. In addition, we evaluated the influence of winter temperatures on the range limit by examining the simulated soil temperatures at the occurrence sites. 3: Laboratory experiments showed that E. longicaudata has a high preferred body temperature and low cold tolerance. The model predicts that maximum activity time decreases with elevation and forest cover. Although unforested areas may provide longer active time in all simulated elevations, mountain areas in Taiwan are heavily forested and are predicted to allow only a very short period of activity above 1000 m elevation. 4: All sightings were indeed located in areas below 1000 m elevation, in which the predicted average soil temperature is above 10 °C in January in cold years. 5: Our results show that reptile physiological response does respond strongly to the change of microclimate induced by forest cover and elevation. Overall, this suggests that forest cover is a major determinant of some reptiles' elevational range.