In order to examine the effect of strain rate on critical stress intensity factors of bones, fracture toughness tests were performed using compact tension specimens prepared from bovine bones under the strain rate ranging from the quasi-static to the high-speed. To confirm anisotropic properties, two different specimens were used, that is, one is the L-direction specimen with initial fissures in the bone running parallel to the axis, and the other is the T-direction specimen with fissures running perpendicular. Further, the specimens under wet and dry states were tested to ascertain the effect of humoral components contained in the bones on critical stress intensity factors. In reference to directional dependence of fissure development, anisotropic properties were observed. Stress intensity factors of the T-direction specimen are two to three times higher than that of the L-direction specimen under the quasi-static range. In the case of wet specimens, directional dependence, however, was not so clearly observed under the high-speed-strain rate compared with the case of the quasi-static range. The physiological state of bone is close to wet specimen compared with dry specimen, and wet specimens possessed a reinforcing mechanism two to three higher in terms of critical stress intensity factor. It was confirmed that the fracture toughness of bone was deteriorated in the vicinity of the strain rate from 100 to 101s-1 regardless of the humoral condition of the specimens. It is speculated that a drastic change of fracture toughness at each strain rate is caused by microfractures due to potential defects at the origin before the main fracture develops.