The critical strain energy release rate (GC) and the critical stress intensity factor (KC) were determined from mode I compact tension testing of human bone. The effect of thickness was also investigated because specimens obtained from human bone do not meet ASTM size requirements due to cortical thickness and curvature of human bone. Calculated values of KC from testing of bovine bone over a range of thicknesses indicated that plain strain conditions were obtained, however, results from GC did not show a similar trend. Reductions in fracture toughness as a result thickness effects were found to be significant. Fracture toughness tests indicated that KC and GC for human bone are approximately 579 N/m and 4.48 MNm-3/2, respectively, for 3 mm human bone specimens. A method of correcting fracture toughness parameters was developed which yielded KC and GC equal to 3.65 MNm-3/2 and 365 N/m, respectively.