Using VC and WC-8Co composite powder, prepared by in-situ reduction carbonization, as raw materials, ultrafine WC-Co cemented carbide was prepared by sinter-HIP technology. The influences of the contents of the VC addition and the carbon of the composite powder on the phase composition, microstructure, and mechanical properties of the cemented carbides were investigated. The results showed that the grain size, hardness and fracture toughness of cemented carbides were mainly affected by the content of VC addition. These properties changed monotonically with the increase of VC content. The change of transverse rupture strength with VC content was related to carbon content. The compression strength decreased first and then increased with the increase of temperature. With the optimized carbon content of WC-8Co composite powder in the range of 5.60-5.68wt.% and the VC addition no more than 0.5wt.%, the high comprehensive performance ultrafine cemented carbides were prepared, with the highest value of TRS at room temperature and compression strength at 600oC as 4482MPa and 4914MPa, respectively. Based on the characteristics of the microstructure and the stress distribution, simulated by the elastic-plastic finite element model, the law of performance change and the mechanism of its performance were analyzed.