Abstract: Objective To study the effect of L2Arginine on the immunity of mouse and its possible mechanisms.Methods Male mice were randomly divided into group A treated with water(for control);group B-E given different concentration of L-Arginine.Mice were divided into normal temprature group and heat stress group.In each group,some mice were killed at the time of 0,2,4,8,12,24 hours after heat stress(41℃±0.5℃,2 hours)treatment,the others were killed at room temperature.Thymus index,spleen index,lymphocyte proliferation,concentration of IL-2 and expression of IL-2R were measured.Results The value of all experimental indexes decreased significantly after heat stress,and reached the lowest at the time of 228 hours after heat stress.With appropriate dose of L-arginine supplementation,thymus index,spleen index,the lymphocyte,proliferation,the level of concentration of IL-2 and expression of IL-2R raised in the group with room temperature;in the heat stress group the level of five indexes decrease significantly.Furthermore,the decrease of the group given 1.5mg/g·bw L-arginine was the smallest.Conclusion The immunity of the mice was damaged to the largest extent at 2-8 hours after heat stress;L-arginine supplementation with 1.5mg/g·bw could protect the immunity of mice after heat stress.