Abstract: Reasons for the strength decrease of P91 steel for steam pipeline in a supercritical unit after service for 88 000 h were investigated from the aspects of martensite substructure, precipitate, solid solution element and dislocation density. The results show that after service the average diameter of M₂₃C₆ carbides in P91 steel increased from 78.0 nm to 190.6 nm, and Laves phase with average particle size of 393.2 nm was precipitated. The contribution value of precipitates to yield strength decreased by 38.7% because of the coarsening of M₂₃C₆ carbides, and the contribution value of Laves phase precipitation to yield strength was very small. The Ostwald ripening of M₂₃C₆ carbides and the precipitation of coarse Laves phase needed to consume C, Cr, Mo and Si elements in the matrix, which reduced the solid solution strengthening effect. The increase in strip block size and strip width of martensite had certain contribution to the reduction of yield strength. The dislocation density of serviced P91 steel was 6.4×10¹³ m^-2, which was lower than that of unserved P91 steel (9.7×10¹³ m^-2); the strengthening effect of dislocation on matrix decreased by 18.8%. Under the effect of all factors, the yield strength of P91 steel after service decreased by 27.0%.