Recently, we found that in ovo feeding of l-leucine (l-Leu) afforded thermotolerance, stimulated lipid metabolism and modified amino acid metabolism in male broiler chicks. However, the effects of in ovo feeding of l-Leu on thermoregulation and growth performance until marketing age of broilers are still unknown. In this study, we investigated the effects of in ovo feeding of l-Leu on body weight (BW) gain under control thermoneutral temperature or chronic heat stress. We measured changes of body temperature and food intake, organ weight, as well as amino acid metabolism and plasma metabolites under acute and chronic heat stress in broilers. A total of 168 fertilized Chunky broiler eggs were randomly divided into 2 treatment groups in experiments. The eggs were in ovo fed with l-Leu (34.5 µmol/500 µl per egg) or sterile water (500 µl/egg) during incubation. After hatching, male broilers were selected and assigned seven to nine replicates (one bird/replicate) in each group for heat challenge experiments. Broilers (29- or 30-day-old) were exposed to acute heat stress (30 ± 1°C) for 120 min or a chronic heat cyclic and continued heat stress (over 30 ± 1°C; ages, 15 to 44 days). In ovo feeding of l-Leu caused a significant suppression of enhanced body temperature without affecting food intake, plasma triacylglycerol, non-esterified fatty acids, ketone bodies, glucose, lactic acid or thyroid hormones under acute heat stress. Daily body temperature was significantly increased by l-Leu in ovo feeding under chronic heat stress. Interestingly, in ovo feeding of l-Leu caused a significantly higher daily BW gain compared with that of the control group under chronic heat stress. Moreover, some essential amino acids, including Leu and isoleucine, were significantly increased in the liver and decreased in the plasma by l-Leu in ovo feeding under acute heat stress. These results suggested that l-Leu in ovo feeding afforded thermotolerance to broilers under acute heat stress mainly through changing amino acid metabolism until marketing age.