Using tungsten oxide, cobalt oxide and carbon black as the raw materials. The nano WC-Co composite powders with pure phases was synthesized by in-situ reduction and carburization reactions. The particle size of prepared WC-Co composite powders is about 80 nm. In this work, the effects of preparation conditions on the phase formation, particle size and oxygen content of the composite powders and on the microstructure and mechanical property of the sintered bulk were investigated. The results indicate that the decarburization reaction will happen in the process of sintering when the oxygen content of the composite powders is higher, leading to a decrease in density and mechanical property of the sintered bulk. The oxygen content declines significantly, with heat treated for two and a half hours, under the heat treatment temperature of 800 _oC. Meanwhile, the mean particle size with heat-treated is 85 nm, without markedly increase. Using heat-treated composite powders with the addition of 1.1%TiC and 0.9%VC grain growth inhibitors, the nano-grained WC-Co bulk by SPS was obtained, with an average grain size of 105nm. The hardness values and fracture toughness obtained are 2145Hv30 and 9.81MPam_1/2 , superior to unheated sample.