Abstract: Objective:to explore C57BL/6N mouse myeloablative irradiation dose,in order to base for founding graft-versus-host disease model of haemotopoietic stem transplantation.Methods: Thirty-three C57BL/6N mice were divided into eight groups according to different irradiation dose,the effects of irradiation on the mice were estimated,and myeloablative irradiation dose was planned.Results: There was difference on white cell count of death between higher than 9 Gy group and 8 Gy group,higher than 9 Gy group died of haemotopoietic failure,and the higher irradiation dose the shorter time of survival,while 8 Gy group survived long time.The most weight lost and the time of arriving to haemotopoietic failure both were different between each group,the higher irradiation dose the more weight lost and the shorter time of arriving to haemotopoietic failure.Conclusion: When rate of irradiation dose is 1.74 Gy/min,the minimum myeloablative irradiation dose of C57BL/6N mice is 9 Gy.