Abstract: A laser confocal microscope was used to study the microstructure variation of P91 heat-resistant steel. The results show that the microstructure starts to transform to austenite (γ) when the temperature rises to 780 ℃. when the temperature reaches 965 ℃, a trace amount of δ-ferrite is emergent. The surface grain boundary of P91 is obvious when the temperature comes up to 1093 ℃, and there is lumphy δ-ferrite generated during heating. A mass of δ-ferrite appear around 1187 ℃, and irregular polygonal grains are present at 1200 ℃.In the cooling rate of 300 ℃/min, the microstructure is δ-ferrite with a spot of precipitates at 1164 ℃. The δ-ferrite to austenitic (α) phase transformation is observed around 909 ℃. This phase transition determines the amount and morphology of the residual δ-ferrite and austenite(γ)phases.