Abstract: Objective:By analyzing the changes of CD4+CD25+Treg level and related cytokines during four-week load-bearing swimming exercise in mice,the effect of aerobic endurance exercise on the CD4+CD25+Treg and related cytokines were discussed.Methods:100 KM mice were divided into EXE group and CON group.Mice in EXE group swam 45 min per day with a load of 5% of body weight,and swam 6 day and rested 1 day per week.Mice were executed before the exercise protocol or 24 h after the last swim every week,and spleen and thymus were taken out.Spleen index and thymus index were calculated,Flow Cytometry were used to measure CD4+CD25+Treg level,and ELISA were used to determine the levels of IL-2,IL-10 and TGF-β.Results:Spleen index increased firstly and then decreased and thymus index decreased with swimming going on in EXE group.CD4+CD25+Treg level in spleen increased compared with CON group(P<0.05),and IL-10 and TGF-βlevels in EXE 3 week group and four-week group increased compare with CON group(P<0.05).Conclusion:Four-week aerobic endurance exercise delayed spleen growth,promoted thymus atrophy and affected the development and function of immune organ.Four-week aerobic endurance exercise induced the activation of T cell,expanded CD4+CD25+Treg,increased the level of IL-10 and TGF-β,and led to immunity drift.