Asymmetric hot rolling was studied in this project as a promising technique for producing steels with fine grain and therefore improved mechanical properties.The asymmetric condition was introduced by applying mismatched roll diameters.In order to apply the results on actual industrial conditions,the diameter ratios were chosen as 1.05 and 1.11.Symmetric hot rolling was also performed for comparison.The rolling temperatures were between 900℃ and 1 100℃.With initial thickness of 10 mm,pass reductions were set at 15%,30% and 60%.The microstructure was observed along the thickness direction.The effect of asymmetric condition on the microstructure evolution in the centre layer of samples is discussed in this paper.The results showed that asymmetric rolling produced higher volume fraction of recrystallization grain and thus smaller average grain size with increasing diameter ratios.At lower rolling temperature,dynamic recovery was the prevailing mechanism.The microstructure of asymmetric rolling distributed more homogenous than that of symmetric rolling.Both force of symmetric and asymmetric rolling decreased with the increasing rolling temperature and descending pass reduction.When other deformation conditions were the same,asymmetric rolling required smaller rolling force than symmetric rolling,except for one condition.Generally,the rolling force deceased with diameter ratio monotonously.