In order to clarify the effects of reduction per pass in non-recrystallization temperature region of austenite (γ ) on ferrite (α) grain size of low carbon steels, isothermal hot compression tests have been performed. The hot deformations have been carried out by the constant reduction per pass of 10, 20 or 30% under the cumulative reduction of 30 or 50% in the non-recrystallization region. The α grain size is decreased about 15% with increasing the reduction per pass from 10 to 30% under the cumulative reduction of 50%. At this time, the α nucleaton site density, which is defined as the number of γ grain boundaries, deformation bands and annealing twin boundaries per unit length of deformation direction, is increased about 30%. It is estimated that the increase in the α nucleation site density is caused by the increase in deformation bands. Furthermore, the number of α nuclei per unit length of γ grain boundaries is increased about 10% with increasing the reduction per pass from 10 to 30% under the cumulative reduction of 30%. It has been clarified by the calculation that the α grain refinement by the large reduction per pass is mainly caused by the increase in the α nucleation site density, which is led by the increase in deformation bands.