The solid solution strengthened P91 steel shows excellent physical and mechanical properties at high temperature. However, degradation in mechanical properties due to softening of martensitic matrix and coarsening of precipitates are still observed. In this paper, initially P91 steel (normalized+tempered) is thermally aged at 650 °C for 4000 h. The microstructural feature and mechanical behavior were investigated for aged sample. The ageing resulted in degradation in mechanical properties. After the ageing, sample was subjected to two heat treatments: (1) tempering (AT), (2) normalizing+tempering (ANT). The main aim was heat treatment to regain the original mechanical properties of the aged P91 steel. The mechanical properties measured for ANT sample was similar to than that of base metal (NT). © 2019 IOP Publishing Ltd.