The fracture mechanics parameters of critical stress intensity factor (Kc) and critical strain energy release rate (Gc) for longitudinal fracture of bovine tibia cortical bone were determined by the compact tension method. It was demonstrated that, for a given bone density, Kc and Gc depended on the loading rate, and resultant crack velocity, with a maximum in fracture toughness (K_c∼ 6.3MNm^−3/2, G_c ∼ 2900 Jm⁻²) at a crack velocity ∼ 10⁻³ ms⁻¹. For a given loading rate, or crack velocity, an increase in bone density, in the range from 1.92 to 2.02 Mgm⁻³, produced increases in K_c and G_c, but a variation in speciment thickness (from 0.5 to 2 mm) had no effect on the measured fracture mechanics parameters.