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 - (2.1∼2.4)Mn - (0.5∼1.0)Si (0.00∼0.02)Nb TRIP steels. The volume fraction of bainite transformed during austempering, ΔVB, and the martensite start temperature, Ms, were measured. ΔVB was dependent on the austempering temperature and a minimum in ΔVB was obtained at 450°C for all the examined steels. Ms temperature was linearly decreased as ΔVB was increased. The ratio of Ms decrease to ΔVB was not dependent on the composition of steels, but on the austempering temperature, exhibiting a maximum when the austempering temperature was 450°C. It was also observed that elongation was the greatest when the ratio was maximum. Ms 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 Ms 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 ΔVB and the Ms.