An experiment involving 18 individually fed barrows weighing about 81 kg was conducted to determine the effects of a high ambient temperature (30°C v. 22°C) and dietary fat supplementation (100 g/kg diet of encapsulated-tallow v. 0 g/kg) on growth performance and carcass fatness of finishing pigs. Digestible energy (DE) intake and growth rates were significantly lower at a high ambient temperature (P < 0·001). Dietary fat supplementation considerably increased DE intakes at a high ambient temperature (40·0 v. 33·8 MJ/day) while slightly increasing it at a thermoneutral temperature (53·1 v. 50·6 MJ/day). Dietary fat supplementation tended to increase growth rates and significantly improved the efficiency of food utilization (P<0·01). Pigs with dietary fat supplementation exhibited greater enhancement in backfat depths, carcass fatness and internal organ fat at a high ambient temperature than at a thermoneutral temperature. Linear correlation between DE intake and crude fat concentration of carcasses was statistically significant at a high ambient temperature (P < 0·01, r = 0·71). Although there appeared to be a positive correlation, it was not statistically significant at a thermoneutral temperature (P > 0·05). Crude fat concentration of the internal organs linearly correlated with DE intake (P < 0·01, r = 0·66). However, fat concentration of the internal organs did not correlate to DE intake at a thermoneutral temperature. The present study indicated that although dietary fat supplementation improved growth performance of pigs over 81 kg live weight initially in hot environments, there was the disadvantage of excessive fat deposition in their bodies.