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.