Effects of Si and Cr contents on bainite microstructure of medium carbon steels have been examined in order to obtain important information for good strength–ductility balanced spring steel. Four practical medium carbon steels, JIS-S55C, SUP9, SUP7 and SUP12 were isothermally transformed at temperatures between 300°C and 500°C after austenitized at 1000°C. The microstructure was observed by means of scanning electron microscopy and transmission electron microscopy. While carbide was formed from the early stage of bainite transformation in S55C without Si and Cr, carbide precipitation was suppressed by the increase of Si particularly in SUP7 and SUP12. The fraction of retained austenite was increased with increasing of Si and Cr at the intermediate stage of bainite transformation because of the increase of C concentration in retained austenite. Particularly, Si addition promoted carbide-free bainitic ferrite and leading to larger amounts of retained austenite by carbon enrichment.