A high speed milling is being received with keen interest in the manufacturing shop where cut hardened alloy tool steels used for metal molds. However, there are some difficult problems, such as high cutting temperature, low stiffness of NC machine, low follow-up ability of NC servo motor etc. This paper deals with cutting phenomena such as cutting temperature, tool life etc. when milling the steels using cemented carbide with grade K10 under various cutting conditions. Furthermore, based on the cutting temperature measured using a work-tool thermocouple technique, the tool wear rate is discussed. The results obtained are as follows. The longest tool life is obtained at cutting speed of 110-130 m/min and feed of about 0.05 mm/tooth. The relationship between work hardness and tool life is linear for each cutting speed on the semilogarithmic coordinate. When the cutting speed is 130 m/min or more, the tool life correlates very closely with the cutting temperature. (Ti, Al)N coated tool has longer tool life of about 2 times and higher thermal resistance of about 100 t compared with carbide tool.