To explain the wear mechanism at the flank face of the cutting tool theoretically, this paper presents some measured data and discussions at the temperature distribution on this face on a single point tool that cuts an ordinary cast iron. When the worn tool cuts the insulated Chrome! and Alumel wires mounted closely in parallel to the workpiece axis, the cut ends of these two wires are joined by a burr produced at one wire and the hot junction of the CA thermocouple is established. The junction, successively, sliding at the worn surface of tool flank, measures the temperature of its rubbing surface. The main results are as follows: 1) The temperature distribution at this surface is almost uniform along the rubbing direction and along the cutting edge except the tool edge point and the boundary of tool-work contact. 2) The average temperature of this face increases exponentially with the cutting speed. And this temperature increases gradually with the worn width up to 1. 6 mm and stays at a constant over it. 3) Under some assumptions, the average temperature at tool-chip interface is estimated from two average temperatures which the one is measured by tool-workpiece thermocouple method and the other is obtained by the above method. Usually, the average temperature at tool-chip interface is higher than that of tool flank.