Abstract: ObjectiveTo study the effects of heat stress on the ultrastructure of thymocyte and the protection function of L-arginine on thymus of rats.MethodsThe ultrastructure changes of thymocyte were observed in rats which were exposed under normal temperature and differenthyperthermia with supplied different concentration L-arginine by gavage.ResultsThe ultrastructure of thymocyte in every group revealled no marked difference at normal temperature condition,but the damages degree of thymocyte ultrastructure was obvious difference in the groups of differentheat-stressed rats.The damages were less in rats with supplementation of L-arginine groups and low temperature group than those with no supplementation of L-arginine groups and high temperature group individually.The ultrastructure changes of thymuocyte at different time after heat stress were different.ConclusionThe differenthyperthermia heat stress could injury the thymocyte and the structure of thymus.As the hyperthermia increase,the damages of thymus aggravated.Adequate supplementary L-arginine could protect the thymus of rats under heat stress.