This study aims to shorten the softening treatment period as possible in high strength structural steels. The steel used is SCM440 steel. As an initial microstructure, martensite, bainite, pearlite and complicated microstructure consisting of ultrafine polygonal, martensite and equiaxed cementite were extensively examined to understand their softening process on aging at 973K. These initial microstructures were prepared by heat or thermomechanical treatment. Their initial Vickers hardness (Hv(10kgf)) were 634, 281, 219 and 238, respectively. It is noteworthy that within five minutes on aging hardness of the complicated microstructure reached lower than Hv200, while it took more than several hours for other initial microstructures. A quantitative evaluation of microstructures appears to help in understanding the mechanism of the softening kinetics.