The formation processes of the retained sustenite(γ_R) in SHCP100 steel sheets were investigated in order to improve the transformation induced plasticity(TRIP) effect of γ_R. An excellent combination of elongation about 23% and high strength over 830 ㎫ was achieved by processing of intercritical annealing and isothermal holding. The mechanical properties of TRIP-aided dual phase steel was found to depend on the volume ratio of each phase and the volume fraction of γ_R. It was also noted that the proper mechanical stability of γ_R improved the mechanical properties. In this work, the best balance of strength-ductility was obtained by holding the steel at 420℃ for 500sec. after annealing at 730℃ for 300 sec.