Improper heat treatment may result in unsatisfactory microstructure and mechanical properties for P91 steel. In this work, the effect of abnormal heat treatment on microstructure and properties of P91 steel were studied. Results showed that with the normalization temperature increased from 860°C to 1000°C, the microstructures of P91 steel are ferrite, mixture of ferrite and martensite, and tempered martensite correspondingly. Cooling rate after austenization has important influence on microstructure and properties. The tempered microstructure has a gradual transition from coarse inhomogeneous ferrite to finer martensite when the cooling rate increased from 1 to 3°C/min. The reduced temperature normalization has no significant effect on hardness even as low as 860°C under aircooling condition, whereas the hareness is inadequate when the cooling rate is too slow. Incomplete austenization and transformation in dual phase zone may account for the phenomenon.