In the present study, evolution of microstructures in intercritical heat affected zone (ICHAZ) of boron added Mod 9Cr-lMo and Mod9Cr-lMo steel is investigated through heat treatment at875°Cfor different durations. Microstructural characterization of these heat treated specimens show that in B added Mod. 9Cr-lMo steels, lath morphology and original prior austenite grain boundaries were present even after prolonged exposure to the intercritical temperature. Whereas, in Mod 9Cr-lMo steel complete grain refinement and recovered lath martensite was observed. The mechanism of reconstitution of prior austenite grain with lath martensite having similar orientation in B added Mod 9Cr-lMo steel is attributed to a phenomenon so called as "austenite memory effect".