Abstract: To observe the effects of thymosin on the expression of HBV DNA，HBV transgenic mice were divided into 3 groups. The animals in group 1 were intraperitonealy injected with native thymosin , while in the 2 control groups with pHGF and saline respectively，for 3 month at a same dose of 3 mg per mouse (injection volumes in 3 groups were 0.75ml) per day. The animal blood was sampled periodically for HBV DNA assay. It was found that the loss rate of HBV DNA in group 1 ranged from 22% (2/9) to 55% (5/9). Twenty two days after the termination of treatment with thymosin,the loss rate remained 22% (2/9),while in the 2 control groups it was 0%, The result suggested that native thymosin could inhibt the expression of HBV DNA in HBV transgenic mice.