Abstract： Objective To investigate the effect of hyperbaric oxygen therapy on T-lymphocyte subpopulations in rats with acute pancreatitis. Methods 56 rats were randomly divided into three groups the sham group ( n = 8 ),control group( n = 24) and hyperbaric oxygen therapy group( treatment group, n = 24), then the control group and treatment groups were divided into three subgroups of 8 rats each undergoing euthanasia on days 1,3,7 after the acute pancreatitis induction. The CD4+ ,CD8+ subpopulations of T-lymphocytes in peripheral blood were detected respectively at the fist day and each day of the euthanasia. The sham group was used to make sure that the model was successfully induced. After euthanasia the pancrea was examined using electron microscopy. Results In the control group, the CD4+ cells in AP rats was significantly decreased and the ratio of CD4+/CD8+ also decreased. After 7days of HBO therapy,compared with the control group, the CD4+ lymphocytes of peripheral blood in the treatment group markedly increased( P ＜0.01 ). The CD8+ lymphocytes also increased to a certain extent. And the CD4+/CD8+ ratio of peripheral blood was obviously increased(P ＜0. 01 ). Also more severe pathological changes appeared in the untreated group than in the treatment group. Conclusion Hyperbaric oxygen therapy could improve the oxygen supply in acute panereatitis, regulate T cell immune function.