Abstract: The effect of strain ageing treatment on the low cycle fatigue (LCF) behavior of P91 martensitic steel at 550 ℃ was investigated by means of fatigue testing, internal friction testing and transmission electron microscopy. The results show that the main reasons that P91 steel happened strain ageing were precipitation of Cottrell air mass made of carbon atoms near dislocations and pinning them. Strain ageing specimen happened remarkable Bauschinger effect under LCF, which reduced the LCF life at high temperature. The increase of tree dislocation density due to strain ageing, which led to form stable cellular structure under cyclic loading, was mainly responsible for Bauschinger effect.