The fracture toughness at crack initiation were determined for bovine cortical bone under tension mode Ⅰ., shear mode Ⅱ., and tear mode Ⅲ. A total of 140 compact tension specimens, compact shear specimens and triple pantleg TP.specimens were used to measure fracture toughness under tension, shear, and tear, respectively. Multiple-sample compliance method was utilized to measure the critical strain energy release rate Gc.at the arWs0.55 crack length, a, to specimen width, W, ratio.. The critical stress intensity factor Kc. was also calculates from the critical loading Pc. of the specimens at the arWs 0.55. The effect of the anisotropy of bone on its resistance to crack initiation under shear and tear loading was investigated as well. Fracture toughness of bone with precrack orientations parallel designed as longitudinal fracture.and vertical designed as transverse racture. to the longitudinal axis of bone were compared. In longitudinal fracture, the critical strain energy release rate Gc. of cortical bone under tension, shear, and tear was 644±102, 2430±836, and 1723±486 Nrm, respectively. In transverse fracture, the critical strain energy release rate Gc.of cortical bone under tension, shear, and tear was 1374±183, 4710±1284, and 4016±948 Nrm, respectively. An unpaired t-test analysis demonstrated that the crack initiation fracture toughness of bone under shear and tear loading were significantly greater than that under tensile loading in both longitudinal and transverse fracture P<0.0001 for all. Our results also suggest that cortical bone has been "designed" to prevent crack initiation in transverse fracture under tension, shear, and tear.