It is well known that machinability of sintered iron is improved by impregnation of resin within the open pores. Embrittlement of the sintered iron by the impregnation as well as decreased friction at the contact surface between the chip and the cutting tool are considered to be the causes for the improved machinability. However, there is room for further study on the cause of the improvement in the machinability. It is known that the cutting temperature of wrought material (conventional full-density material) is lowered with increasing thermal properties such as thermal diffusivity α and value of λρc, where λ is the thermal conductivity, ρ is the density and c is the specific heat of the work material. The purpose of this study was to elucidate the effect of resin impregnation on the cutting temperature, and therefore the thermal properties of impregnated sintered iron were investigated. It was found that the thermal properties were lowered by the resin impregnation. However, the degree of reduction in the thermal properties was less than 10%. The experimental results suggest that the small change in thermal properties due to the resin impregnation is not the main cause of the drastically improved machinability.