Hafnium carbide/tungsten (HfC/W) cermets were prepared by an in situ reaction sintering process, using hafnium oxide (HfO2) and tungsten carbide (WC) as the raw materials. The reaction path, densification behavior, microstructure development, and mechanical properties of the cermets were comprehensively investigated. It was found that WC decomposed to tungsten semicarbide (W2C) and tungsten (W) in sequence, and meanwhile HfC was formed by carbothermal reduction between HfO2 and as-released carbon from the dissociation of WC. The solid solution formation between HfC and W during sintering was also studied. The obtained cermets (>98% TD) have a Vickers' hardness of 8.16 GPa, a fracture toughness of 14.45 MPa m1/2, and a high flexural strength of 1211 MPa.