WC matrix composites toughened by commercial Al2O3composed of AlOOH, χ-Al2O3and amorphous Al2O3were prepared by high energy ball milling and then hot pressed at 769°C, 984°C, 1100°C, 1540°C for 90min, respectively, to investigate the phase transformation process. Microstructure and mechanical properties of WC-40vol.%Al2O3composites were studied as well. The results showed that, with the increasing hot pressing temperature, the commercial Al2O3 transformed to α-Al2O3completely through γ-Al2O3. WC could be oxidized and decarburized to W, C and W2C at low temperature when the vacuum degree was very low. The existence of W in the composites sintered at 1540°C for 90min could contribute to a metal particle toughening effect to the composites, resulting in the achievement of an excellent fracture toughness of 10.43MPa•m1/2, combining a Vickers hardness of 18.65GPa with a relative density of 97.98% for WC-40vol.%Al2O3composites.