Objective: To study the effect of natural killer (NK) cells on graft-versus-host disease (GVHD) after H-2 haploidentical bone marrow transplantation (BMT) in mice. Methods :Murine model of H-2 haploidentical BMT was established by using Balb/c (H-2d) mouse as recipient, and Balb/c(H-2d)×C57BL/6 (H-2b) (H-2d/b) mouse as donor. Lethally irradiated Balb/c (H-2d) mice were transplanted with the bone marrow cells from Balb/c(H-2d)×C57BL/6(H-2b) (H-2d/b) mice containing donor spleen cells and/or NK cells. GVHD and survival rates were studied by observation of clinical manifestations and pathological changes. Results:In the group of bone marrow +spleen cells, GVHD was induced in 90% mice; but in the group plus with low amount of NK cells,GVHD was induced in 20% mice; and in the group transplanted with high amount of NK cells, GVHD was induced only in 10% mice. Compared to the group transplanted only with BM plus spleen cells, the incidences of GVHD in the latter two groups decreased significantly (P＜0.01) and the survival rates at different periods of 15, 30, 45 and 60 days increased obviously (P＜0.01 ). Conclusion: In mouse H-2 haploidentical BMT, alloreactive NK cells can reduce the incidence of GVHD and increase the survival rate.