The cutting temperatures on the tool face are very important factors in machining as well as cutting force and chip formation. But the measurement of cutting temperatures of diamond tool is impossible with the conventional work-tool thermocouple technique, because diamond is an electrically non-conductive material. In this report, a calibration equation is firstly obtained from the relation between the cutting temperatures with work-tool thermocouple technique and the cutting temperatures with chip-chip breaker thermocouple technique using cemented carbide tools which are the electrically conductive materials. And the cutting temperatures of diamond tool are estimated by substituting the value obtained from the chip-chip breaker thermocouple technique with diamond tools into the calibration equation. The results show that this method is useful to suggest the cutting temperatures of diamond tool.