ABSTRACT Purpose: This study compares the effects of high linear energy transfer (LET) fast neutrons on the induction of bone loss in mouse tibia with those of low LET (60)Co γ-rays. Materials and Methods: The mice that had received 0-6 Gy of γ-rays or 0-2.4 Gy of neutrons were examined 12 weeks after irradiation. Tibiae were analyzed using microcomputed tomography. Serum markers, biomechanical property, and osteoclast surface were measured. Results: Micro-computed tomography revealed that proximal tibial metaphysis from irradiation group had lower trabecular bone compared to the sham group. Bone loss was markedly enhanced by irradiation. Mean neutron relative biological effectiveness values (Ref. gamma) of 2.05 and 2.33 were estimated for irradiation of trabecular bone volume fraction and of bone mineral density, respectively. There was a substantial reduction in osteoclast surface of tibial metaphyses in irradiated mice with high dose of neutrons. Conclusions: There was a significant relationship between the loss of bone archtecture and the radiation dose. Neutron irradiation had a more marked effect on osteoclast activity than gamma irradiation.