The change of the number of M sub(23)C sub(6) particles intercepting mobile dislocations by creep of high-chromium creep resistant steel is deduced. With tempering at 800 degree C, the number of intercept particles decreases slower than the total number of particles. The dissolution velocity of particles is greater by smaller particles. By tempering at 550 degree C and higher, the dissolution of small particles increases the intercept particles spacing and the creep rate for about 1 order of magnitude stronger than the increase of average particles size. M sub(23)C sub(6) particles size, number and dissolution velocity were deduced for tempering of a X20 steel at 800 to 550 degree C. The dissolution of small particles increases the particle spacing for above one order of magnitude more than particles coarsening. By creep tests above 550 degree C, particles spacing and creep rate are increased significantly by dissolution of small particles.