Abstract: The behavior of WC particles of WC particle-reinforced M2 high speed steel composites（MMCs） made by spray formming with emphasis on the change of prtculate composition during the course of deposition was investigated. The dissolving-precipitating phenomenon of WC particles contacted with high-temperature liquid high speed steel was studied.The results showed that particulate composition of WC particles changed fundamentally during preparationof MMCs. In comparison with compositon of WC grain of traditional hard alloys and steel bending hard alloys, a dissolving-precipitationg mechanism interpreting the particulate changing characteristicewas proposed.