The histological changes of bone defect sites created surgically in rat femur treated by low level laser therapy (LLLT) are evaluated. Thirty rats are randomly divided into 3 groups according to different methods of laser irradiation: multi-irradiated group, single-irradiated group and control group with no irradiation. The total doses of both of the irradiation groups are the same. A round bone defect is created on the femur of each rat. The rats are killed on the 15th day and the 30th day after the surgery. The results show that after drawing materials and HE staining, the inflammation and bone formation of each group are analyzed under an optical microscope. The inflammation scores of irradiated groups are lower than those of control group with significant difference, meanwhile, the bone formation scores of irradiated groups are higher than those of control group with significant difference. However, there is no significant difference between the two irradiation groups. It is concluded from the study that LLLT with proper dose may alleviate inflammation and promote bone formation. There is no difference between multi-irradiated group and single-irradiated group.