In experiment one, two levels of vitamin A supplementation (3,000 and 9,000 IU/kg) were used to investigate the laying performance and antibody titer against Newcastle disease virus of heat-stressed hens. Results indicated that the higher supplementation level had a beneficial effect on the feed intake and laying rate of heat-stressed hens compared with the control group with the lower supplementation level. Antibody titers were not affected by the level of vitamin A. In experiment two, four levels of vitamin A were given (3,000, 6,000, 9,000 and 12,000 IU/kg) to test their effects on the antibody titer to Newcastle disease virus and the T lymphocyte proportion. The experimental birds were exposed to a high temperature (31.5 A?C) 15 days after vaccination against the Newcastle disease virus (Treatment 1) or immediately (Treatment 2). Results indicated that egg weight was increased in response to the high levels of vitamin A supplementation (6,000 and 9,000 IU/kg), but feed intake, laying rate, and body weight loss were not. Vitamin A had no significant effect on antibody titers against the Newcastle disease virus in normal or hot environments in Treatment 1, but the proportion of A?-naphthyl acetate esterase-positive cells increased. Vitamin A supplementation had significant effects on the Newcastle disease virus antibody titer and the A?-naphthyl acetate esterase-positive cell proportion in Treatment 2. Conclusions stated that vitamin A supplementation in commercial layer diets to layer chickens under heat stress was beneficial to laying performance and immune function.