cBN-WC-12Co cemented carbides were prepared by spark plasma sintering (SPS) technique in this paper. The thermal stability of cBN was analyzed. The effect of cBN content and sintering temperature on the hardness and density of the cemented carbides were discussed, and the strengthening mechanism of cBN was discussed. The results show that the thermal stability temperature of cBN is 1335℃. The lowest dense sintering temperature of cBN-WC-12Co cemented carbides is about 1150℃, and cBN keeps an ideal thermal stability with no phase transition at 1250℃. The density of cBN-WC-12Co cemented carbides is better than WC-12Co at the same sintering temperature; The alloy C15 sintered at 1150℃ is with the highest density and hardness (HV10), which are 99.7% and1997,respectively; The increase of cBN content is beneficial to inhibit the growth of WC grains, to a certain degree, when the content of cBN is lower than 15vol%. The smaller WC grain is helpful to promote the densification of the cemented carbides through its capillary adsorption ,while the cBN can prevent the spread of crack and other defects, then to improve the mechanical properties of the cemented carbides.