Abstract: The phase constituent and structure of a Fe-W(Mo)-Co(Ni) age hardening tool steel were investigated by TEM-SEM, X-ray and image analysis. The mechanizm of age hardening indicated that the non-barrier precipitation-spinodal decomposition happened at the initial stage of age hardening to form a period structure, the growth of this structure was very slow, and it caused the fast hardening and slow softening in the aging process.