Cold rolled TRIP-600 steels has been developed on a laboratory scale. Composition of the steels is 0.12wt%C-0.30wt%Si-1.6wt%Mn-1.2wt%Al. Effects of Al concentration on phase diagram, TTT curves and T0 line of bainite formation in the steels were calculated and discussed.Cold rolled sheets of 1.7mm thick were annealed between 830 °C and 870 °C followed by austempering at 420 oC. UTS of 660~700MPa, YS 390~420MPa and total elongation 30.5~37.5% is obtained.Volume fraction of the retained austenite is 9~11% and carbon content in the retained austenite is 0.9% ~1.3wt% in the steels. The chemical composition and processing parameters of the experimental steels could be referred for trial-manufacture of TRIP 600 steels.