Medium carbon steels have low yield ratio (YR). In order to obtain high yield strength (YS) for them, different microstructures including ferrite(F)/cementite(C) and ferrite(F)/pearlite(P) are prepared and the microstructure for high YR as well as its mechanism is clarified. The purpose of this study is, therefore, to elucidate the effect of ferrite grain size on YS in the relationship between F/P and F/C structures having the same tensile strength (TS) level. In case of F/C, YS can be significantly improved through grain refinement with a slight decrease in ductile properties. This is particularly clear by comparison of F/C with a ferrite grain size of 0.6m and F/P, where both structures show the same TS level while the YS level is significantly different; F/P representing much smaller YS than that of F/C. It is also to be noted that F/P microstructure shows low YS compared to its high TS with a generation of dislocations at the interface between ferrite and cementite in its pearlite phase. In conclusion it is necessary to consider the difference in the YS-TS relationship between F/C and F/P in order to make the most of its forging processing.