In order to optimize the strength of P92 heat-resistant steel, the variation in hardness and microstructural evolution during creep were investigated. The results show that before crept for 1429 h at 873 K, the coarsening of M23C6 is the main factor to decrease the hardness. When the creeping time prolongs from 1429 to 6063 h, the increase of hardness is mostly attributed to the precipitation of a large amount of Laves phase. Thereafter, the coarsening of Laves phase leads to the decline in hardness. The precipitation hardening resulting from MX drops distinctly in spite of the slight growth during creep. It is important to control the growth of MX, decrease the coarsening velocity of M23C6 and keep Laves phase to be fine.