Grain size refinement can be achieved by heavy deformation of the HSLA steel. Ferrites nucleated during hot working was defined as strain induced dynamic transformation(SIDT) ferrite and reported as an important factor to refine the final ferrite grains. The effect of the prior austenite grain boundary and deformation band area on the transformation from austenite to SIDT ferrite was investigated. The Fe-0.15C-1.1Mn-0.25Si-0.04Nb-0.002B steel was vacuum induction melted and size-rolled to 15mm thickness sheet. After austenitizing heat treatment at 1200degreesC for 1 min., two step hot compression were conducted by Gleeble 1500. Primary compression was conducted at 900degreesC in non-recrystallization region with the strain ranged from 0.2 to 0.6. Secondary compression was conducted at 650degreesC with a strain of 0.4 in the under-cooled austenite region. Microstructural parameters, such as prior austenite grain boundaries and deformation bands, were sensitively dependent on the imposed strain during the primary compression process. SIDT ferrites were nucleated at prior austenite grain boundaries and deformation bands during second compression process. Grain boundary area and deformation band area before secondary compression were measured by line intercept method through the microstructural observation and the effective surface area was calculated by adding the grain boundary area and the deformation band areas Grain boundary area with different strain conditions in the primary compression was predicted by the suggested model. As the strain of primary compression increased, effective surface area increased and nucleation of SIDT ferrite was enhanced with increasing the effective surface area. It was concluded that transformation from austenite to SIDT ferrite is diffusion-controlled.