9Cr ferritic/martensitic (F/M) steels with 0 and 0.3wt.% yttrium (Y) addition were fabricated via vacuum casting technique and were creep tested at 650°C and 120MPa. The creep property of 9Cr F/M steel was improved by the addition of yttrium. Even though the initial creep rates of 9Cr-0Y and 9Cr-0.3Y were almost the same, the creep rupture life of 9Cr-0.3Y specimen was extended about 2˜3 times with the longer steady state creep stage and the lower minimum creep rate than 9Cr-0Y specimen. The coarsening of grains and martensitic laths occurred during creep, and M23C6 carbides, MX/M2X-type carbonitrides and Fe2W/Mo Laves phase were identified along the boundary in both specimens. For 9Cr-0.3Y, the nano-sized oxide clusters were formed during creep, because of the diffusion and local segregation of the dissolved yttrium and oxygen atoms at high temperature and with the promotion of creep stress. The softening of microstructure recovery is compensated by the dispersion strengthening of nano-particles and the balance of them was the reason of longer steady state creep stage of the creep curves.