Abstract： Objective To observe effect of human bone marrow mesenchymal stem cell (MSC) on T cell activation in patients with aplastic anemia(AA). Methods MSCs were separated with Percoll and cultured in low glucose DMEM. T cells were harvested by using nylon column. MSCs of various concentrations were added to PHA induction T cell activation cultures. The CD25 and CD38 expression rates of CD3+ T cells was analyzed by flow cytometry. Results MSCs were planted in 96 well plates (1×104/well for group A) and coeultured with T cell isolated from peripheral blood of AA patients. Peripheral blood T cell noncocultured with MSC acted as the control group, MSCs inhibit PHA induced the CD25 and CD38 expression of CD3+ T cells in patients with AA. The expression rates were significantly lower than control group (P<0.01). But the expression rates showed no significant difference between the two groups(MSCs , 1×103/well for group B). Conclusion Human bone marrow MSCs inhibit T cell activation in patients with AA in vitro and the inhibitory effects are dependent on the amount of MSCs.