Variously heat treated tool steel SK 3 and high speed steel SKH 9 were machined with sintered CBN tools. Tool wear and cutting temperature were examined with regard to work hardness. Further, diffusion tests were conducted between steels and CBN grains or CBN tools to ascertain the possibility of chemical wear of CBN tools. The results obtained were as follows:(1) Extremely large wear of conventional CBN cutting tools in machining of low hardness work materials is not the inherent chemical characteristic of CBN itself but the characteristic of mechanical properties of the tools depending on sintering mechanism.(2) The average cutting temperature of CBN tool is fairly lower than that of carbide tool.(3) When the hardness of work materials exceeds a certain limit, the chip becomes thin and sawtooth shape, and the cutting temperature tend to decrease with the increase of work hardness.(4) Around the temperature of 1200°C, CBN grains are fairly stable for the reaction to Fe. But when they are sintered, B or Co around the tool surface tends to diffuse into Fe.