Abstract: Behaviour of hard phase of steel bonded WC hard alloy during heat treatment has been investigated. Qualitative and quantitative analyses of their phase compositions were carried out with electron probe and scanning electron microscope. Phase structure analysis and metallographic structure observation were made by X-ray diffractometer and optical and electron microscope respectively. During heat treatment the solution and precipitation between hard phase and binder phase of steel bonded WC hard alloy have been proved. The influence of the change of chemical compositions in steel binder phase on heat treatment conditions, structure and properties is discussed, so that the theoretical bases of the heat treatment of this kind of materials can be given.