Effects of Mo and W on creep strength of low Cr heat resistant steels have been investigated from the stand points of the phase stability of precipitates during long-term exposure at the elevated temperatures. The study on 2.25%Cr–Mo–V–Nb (Mo-steel) and 2.25%Cr–W–V–Nb (W-steel) with a same amount of Mo-equivalent has shown that the substituting W for Mo retards the evolution of microstructure, and thus remarkably improves the creep rupture strength. TEM observations have suggested that the most important precipitates strongly affecting the stability of microstructure are M 6 C type carbide and MC type carbide in both Mo-steel and W-steel. It is found that the M₆C carbide precipitates with concentration of Mo and W during long-term aging. Therefore, the amounts of solute Mo and W supersaturated in matrix have reduced to the thermal equilibrium levels with changing the substructure from bainite lath structure to sub-grain liked structure. The kinetics of Mo- and W-partitioning between matrix and M₆C carbide has been successfully expressed by a Johnson-Mehl-Avrami type equation and estimated the activation energy for diffusion of solutes; 125 kJ/mol in Mo-steel and 219 kJ/mol in W-steel. Thus the diffusion rate of W is suggested to be definitely lower than that of Mo. Furthermore, Mo-and W-partitioning to MC carbides have also been confirmed to affect the coherency and the growth rate of MC carbides. TEM observations on the strain image have shown that the MC carbide in Mo-steel has already lost the coherency with matrix. On the other hand, the finer MC carbide in W-steel has kept the coherent relationship with matrix even after long-term aging. It is concluded that W-steel is superior to Mo-steel in the stabilization of bainite lath structure and precipitates at elevated temperatures, resulting in the higher creep strength.