Effects of surface area of prior austenite grain boundaries and deformation bands on austenite/ferrite phase transformation of high strength low alloy (HSLA) steel during hot working were investigated through simulated hot workings using Gleeble 1500. Effective surface areas were controlled by applying different austenite conditionings. It was found that the volume fraction of strain induced dynamic transformation (SIDT) ferrites increased with an increase in the effective surface area. The volume fraction of SIDT ferrite nucleated from the elongated prior austenite grains, however, was higher than that nucleated from equi-axed prior austenite grains at the same effective surface area. From the experiment results and discussions, it was concluded that diffusional austenite/ferrite phase transformation of HSLA steel by the interface nucleation and growth mechanism is accelerated by the deformation bands in the non-recrystallization region.