The coarsening behavior of martensite lath has been investigated by means of transmission electron microscopy for tempered martensitic 9 wt.% Cr–(0, 1, 2, 4 wt.%) W steels during creep at 823–923 K. During creep, the recovery of excess dislocations, the agglomeration of carbides and the coarsening of laths take place. The coarsening of laths with absorbing excess dislocations is the major process in the creep acceleration. The coarsening rate of lath decreases with increasing W concentration, which is correlated with the rate of Ostwald ripening of M 23C 6 carbides. The progressive local-coalescence of two adjacent laths boundaries near the Y-junction causes the movement of Y-junction, resulting in the coarsening of lath.