The early RBE of the bone marrow in mice after single irradiation with fast neutrons (35 MeV_p→Be ) was studied. ⁶⁰Co-γ ray was used for referent beams. Using the dose making 50% loss of the nucleated cells of bone marrow in mice relative to control group mice to calculated the RBE value which was 2.13±0.18. Meanwhile, the relationship of the RBE values and the dose of neutrons was noted. On log-log plot, the RBE values decrease with increasing dose of fast neutrons and it is consistent with a slope of -0.39± 0.10. The α/β ratios were estimated from linear-quadratic model of cell survival, they are 14.4±1.30 Gy for fast neutrons and 0.83±0.10 Gy for γ-ray, respectively.