Abstract: The effects of deformation at 900℃ and 780℃ on the microstructure and mechanical properties of the bainitic ferrite formed by isothermal treatment have been studied. The samples include three steels:an ultra-low carbon microalloying clean steel containing Mn, Nb, Ti, B, commercial steels X60 and XTE355. After press 70% at 900℃ or 780℃ followed by isothermal holding at 500℃ for 500 s or 800 s, samples of all three steels showed very fine structure and great improvement in their strength.The smallest overage grain size, which is 0.99μm and 1.02 μm along X and Y direction respectively, is obtained in a X60 specimen pressed 70% at 780℃ before isothermal holding.But the greatest improvement in strength is shown in the clean steel specimen owing to its large fraction of bainite structure. Mechanisms affecting microstructure and strength in the steels,which were deformed at near A_r3 or between A_r3 and A_r1 temperature before isothermal transformation, are discussed.