Microstructure evolution under long-term aging and creep was studied in a 9wt%Cr-3wt%Co-3wt%W martensitic steel at a temperature of 650 A degrees C and stress ranging from 100 to 220 MPa with a step of 20 MPa. This steel exhibited creep strength breakdown at an applied stress of 160 MPa and a rupture time of 1703 h. However, this creep strength breakdown did not coincide with the transition from short-term creep conditions to long-term creep, because deviation from the Monkman-Grant relationship occurs at a minimal strain rate of similar to 3 x 10(-6) h(-1), and the acceleration of the creep rate by strain, dln /d epsilon, in the acceleration region at applied stresses of 120 and 100 MPa significantly differs from the acceleration at greater applied stresses. The transition from short-term creep to long-term creep correlates with the strain-induced coarsening of the M23C6 carbides and the Laves phase particles, which leads to dissolution of the fine particles and the growth of coarse particles of these phases at the lath boundaries. With a decrease in the applied stress, the overall Zener drag force exerted by the boundary particles decreases below the critical value of 0.12 MPa, and the tempered martensitic lath structure transforms to a subgrain structure.