Tensile and dilatation tests were conducted in order to study the effects of Mn, Si and Nb on the transformation behavior in the 0.14C–(2.12.4)Mn–(0.51.0)Si–(0.000.02)Nb TRIP steels. All the specimens for dilation tests were annealed under the condition to form the same austenite fraction, followed by cooling to the austempering temperature. The volume fraction of bainite transformed during austempering, ΔV_B, and the martensite start temperature, M_s, were measured. ΔV_B was dependent on the austempering temperature and a minimum in ΔV_B was obtained at 450°C for all the examined steels. The M_s temperature was linearly decreased as ΔV_B was increased. The decreasing rate of M_s to ΔV_B was not dependent on the steel composition, but on the austempering temperature, exhibiting the maximum value when the austempering temperature was 450°C. It was also observed that volume fraction of retained austenite was linearly increased with the decrease of M_s for the steels austempered at various conditions. The highest elongation was obtained when the rate of M_s to ΔV_B was the maximum. M_s was increased when the amount of Mn was increased from 2.1 % to 2.4 %. It was because Mn retarded the ferrite transformation rate, so that the soluble C diffused from the ferrite to the austenite was decreased. The addition of Si decreased M_s because it activated the diffusion of C to austenite during the cooling to the austempering temperature. The addition of Nb was observed to give little influence on ΔV_B and the M_s .