This study investigated the time course of changes of bone and muscle atrophy in mice with immobilization by denervation and fixation. The animals were fifty-two male C57 BL/6J mice, aged 10 weeks old. Eight mice were used as the base line, and the remaining ones were cut at the sciatic nerve of the left hind limb and fixed with a plaster cast. At week 1, 2, 3, and 4 after the operation, a cross-sectional area of the rectus femoris muscles and bone mechanical strength with a three-point bending test of the femur and tibia were measured. The time course of changes of the bone mechanical strength and of the cross-sectional area of the rectus femoris muscles between the intact and experimental limbs in each period compared with the control limbs, was determined. The bone mechanical strength of the femur, tibia, and the cross-sectional area of the rectus femoris muscles of the experimental limbs significantly decreased compared with those of the intact limbs at week 4, 3, 2 and 1 after the operation (p<0.05). Compared with the intact limbs, the bone mechanical strength and the cross-sectional area of the rectus femoris muscles of the experimental limbs declined approximately 10% and 30%, respectively, during the experiment (p<0.05). It was demonstrated that bone and muscle atrophy occurred at an early stage after immobilization by denervation and fixation, and that both types of atrophy progressed simultaneously in the present study.