The comparative study of high temperature deformation behaviour of P91, RAFM-CLAM and Low C RAFMCLAM steels was done from compression deformation data in the temperature range of 850–1100 °C and the strain rate range of 10−3–10s-1. The processing maps in terms of strain rate sensitivity (m), % efficiency of power dissipation (Æž) and the instability criterion (ξ(Î)) were constructed in the temperature-strain rate space. The stable domains for hot processing with higher m and % efficiency values were identified. For P91, the stable domain was observed in between 970 °C–1100 °C at 0.01–1 s-1 strain rate with maximum m value of 0.223 and 36% efficiency. For RAFM-CLAM steel, the stable domain was observed in between 1000 °C–1050 °C at 0.3-5 s-1 strain rate with maximum m value of 0.189 and 30% efficiency. For Low C RAFM-CLAM steel, the stable domain was observed in between 900 °C–1000 °C at 0.01–0.1 s-1 strain rate with maximum m value of 0.175 and 30% efficiency. The unsafe conditions for industrial hot working of the steels were determined from the negative values of ξ(Î). The detailed kinetic analysis including activation energy and activation volume was carried out. The rate controlling mechanisms based on activation energy and activation volume values were concluded to be movement of screw dislocations and thermal activation breaking of attractive junctions.