Effects of W and Co on microstructure of Co free 0.1C-15Cr-1Mo-3W-0.2V-0.05Nb-0.07N-0.003B (mass%) steel has been investigated, and a correlation between microstructure and creep strength property has been discussed. Time to rupture of 6W-3Co steel, in which W content was increased from 3 to 6mass% and 3mass%Co was added, was about 100 times longer than that of 3W-0Co base steel at 923K-100MPa and was almost the same as that of the conventional tempered martensitic creep resistant steel of NF616 steel. It has been found that the increase in W content and the addition of Co increases a number of coarse block type precipitates and promotes a precipitation of fine particles within grain, respectively. Furthermore, a large number of needle or disk-like fine particles aligned in a certain crystallographic orientation within grain has been observed in the creep ruptured steel containing high W and Co. It has been concluded that a remarkable increase in creep strength by the addition of W and Co is obtained by a combination of increase in an amount of coarse precipitates and a distribution of needle or disk-like fine particles within grain.