Tungsten carbide/high strength steel (WC/HSS) composites were prepared by cold pressing and vacuum sintering. The microstructure and micro-hardness of the composites with different particle sizes were analyzed by optical microscopy (OM), scanning electron microscopy (SEM), ultra-high temperature laser confocal microscope and microhardness tester. The results show that WC density tended to increase as the average grain size of WC decreased. When the particle size of WC powder was 100 nm, the density reached 91.22%. The micro-hardness of WC increased with the decrease of WC particle size. When the particle size of WC powder was 100 nm and sintering temperature was 1 320 ℃, the micro-hardness of WC reached about 1680 HV0.1. The micro-hardness of WC near the bonding interface was higher than that of other parts. The microhardness of HSS core varied little with WC particle size, which was about 500 HV0.1. When the particle size of WC powder particles was 200 nm, a transition layer with a certain width was formed at the interface between WC and HSS, and the combination between the two materials was metallurgical. The iron element in the HSS matrix diffused into WC structure contacted with it, resulting in a certain width of a fusion layer, and the composite interface was relatively well bonded. When the average particle size of WC powder was 200 nm, W, Fe and Co elements significantly diffused in the transition zone at the interface. With the increase of WC particle size, the trend of elements diffusion decreased. When the particle size of WC powder particles was 500 nm, no obvious diffusion of Fe and W elements occurred at the interface of the composite, but only a certain degree of diffusion of Co element occurred.