The formation processes of the retained austenite(${gamma}_R$) in SHCP100 steel sheets were investigated in order to improve the transformation induced plasticity(TRIP) effect of ${gamma}_R$. An excellent combination of elongation about 23% and high strength over 830 MPa 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 ${gamma}_R$. It was also noted that the proper mechanical stability of ${gamma}_R$ improved the mechanical properties. In this work, the best balance of strength-ductility was obtained by holding the steel at $420^{circ}C$ for 500sec. after annealing at $730^{circ}C$ for 300 sec.