Abstract: The precipitate phase transformation in P92 steel during isothermal aging at 650 ℃ for different times was investigated by physicochemical phase analysis and XRD analysis. The results show that the precipitates in P92 steel under the as-received condition primarily consisted of M23C6 carbides and MX phase. With the prolongation of the aging time, the total precipitation content increased continuously. The content of M23C6 increased slightly, while Laves phase nucleated and grew rapidly during aging. After aged for 3 000 h, the content of M23C6, MX and Laves phase seemed to be stable. Laves phase content plays a key role in the variation of total precipitation content.