The mechanical properties of O.6C-Si-Mn steel transformed isothermally in the bainitic temperature region (593 and 648 K) were investigated. The mechanical properties of the steels were improved with increasing bainite and retained austenite and the corresponding decrease in martensite. Marked benefits of the mechanical properties were obtained for the steels containing the maximum content of retained austenite in the bainite matrix, independent of transformation temperature. For isothermal transformation at 593 K, the 0.2% yield stress, sigma(y), ultimate tensile stress, sigma(u), and notch tensile stress (NTS) were improved significantly, while the advantage of the per cent elongation and Charpy 2 mm V-notch (CVN) impact energy was relatively small. As a result of isothermal transformation at 648 K, the per cent elongation and CVN impact energy were dramatically improved, while the superiority of sigma(y), sigma(u), and NTS was not much greater than isothermal transformation at 593 K. Compared to 0.6C steels transformed isothermally at the same temperatures, in which little appreciable retained austenite was found, the isothermally transformed steels having a microstructure consisting of bainite and retained austenite improved the mechanical properties remarkably. These results are described and discussed.