Abstract: The oxidation and decarbonisation behavior of high speed steel W9, M2, M2Al and D606 heated in the air were studied. The results showed that the weight loss by oxidation and depth by oxidation of four high speed steel increase in the air, When the temperature and times increase. The reasons of oxidation and decarbonisation behavior were analyzed. Influence of Si, W, Al elements on oxidation and decarbonisation of high speed steel were discussed.