9Cr3W3CoB steels are developed to serve at the temperature range of 620-650℃,and have been recog-nized as the most promising martensitic heat-resistant steels for supercritical power plants.Due to the high W and Co contents,the Fe2W Laves phase in such 9Cr3 W3CoB steel possesses some specialties in thermodynamics.In the present research,it was found that even when aged at 800℃in the 9Cr3W3CoB steel,instead of dissolving,Laves phase formed after 50 h and kept on increasing in size and number density until 1000 h,indicating that the Laves phase was marching for the thermodynamic equilibrium during aging.In this thermodynamic process,the W-rich M3B2 borides in as-received steel and M23C6 carbides were revealed to dissolve,supporting the growth of Laves phase.SEM investigation indicates that Laves phase tended to form clusters,and finally grow as a unit bulk Laves phase with an irregular shape.Besides,Laves phase nucleated next to M23C6 carbides and enwrapped them inside at 800℃.In addition,the growth processes of Laves phase and M23C6 carbides were a competitive procedure,the coarsening of M23C6 carbides was prior to the growth of Laves phase at 750℃while the growth of Laves phase was prior to the coarsening of M23C6 carbides at 800℃.