Abstract: We observed the histological changs of thymus in rats which were exposed under normal temperature and different hypert hermoa with supplied different concentration arginine by gavage.The results showed that the structure of thymus in every group revealled no marked difference at normal temperature condition,but the damages degree of thymus structure was obvious difference in the different heat-stressed rats.The damages were less in rats with supplementation of arginine groups.low temperature groups and short time heat-stressed groups than those with no supplementation of arginine groups,high temperature groups and long time heat-stressed groups individually.And that the histological changes of thymus followed the different time after heat stress were different.It suggested that the different hypert hermia all could injury the cells and structure of thymus.As the hypert hermia increased and heat2stressed time prolonged,the damages of thymus aggravated.Adequate supplementary arginine could protect the thymus of rats under heat stress.