Abstract: The P92 steel was aged at 650 ℃ for different periods of time (200-5 000 h) in a isothermal aging test for simulation of actual working conditions. The formation and coarsen behavior of Laves phase were investigated using SEM and TEM. The results indicate that Laves phase formed when the specimen was aged up to 200 h, and the size of Laves phase continued to increase with the prolongation of aging time. After being aged for 2 000 h, the Laves phase aggregated and coarsened obviously, and average size of Laves phase was the largest in P92 steel.