Objective To study the effects of thymus transplantation(TT)combined with CD4--DLI on T cell reconstitution after allogeneic hematopoietic stem cell transplantation(allo-HSCT). Methods BALB/c mice were randomly divided into three groups:hematopoietic stem cell transplantation (HSCT group),hematopoietic stem cell transplantation combined with thymus transplantation(TT group)and hematopoietic stem cell transplanta-tion combined with thymus transplantation plus CD4+ T cell-depleted lymphocyte infusion(CD4--DLI group). On day-1,the mice were treated with the lethal dose of radiotherapy. On day 0,C57BL/6 mice were used as donor for hematopoietic stem cell transplantation. The mice were sacrificed on 5 days,2 weeks,4 weeks and 3 months after transplantation,respectively. The peripheral blood and spleen cells of mice were collected for determinations of T cell surface antigen,T cell receptor,naive T cells and intracellular cytokines. HE staining was used to assess the development of donor thymus. Results TT and CD4--DLI did not impair each other′s effects on T cell reconstitu-tion. TT combined with CD4--DLI increased the number of T cell reconstruction. CD4--DLI promoted the effect of TT on enlargement naive CD4+and CD8+T cell pool. Combination of TT and CD4--DLI enhanced the cytokine pro-duction of T cells. Conclusion TT combined with CD4--DLI had no side effects on TCR repertoire and thymus. Conclusion TT combined with CD4--DLI can enhance the reconstitution of T cell number and function via thymus dependent and thymus independent mechanism.