In this paper the effect of interstitial heat treatment on the microstructure and mechanical properties was examined both in the 0.15C-6Mn steels and 0.15C-6Mn steels added with Nb or Ti. This result will be applied into the development of a steel which has the properties of high strength and high ductility resulted from the transformation induced plasticity. The strength-elongation combination was increased as the holding time was increased when the temperature is at $625^{circ}C$. However, the strength-elongation combination was decreased sharply as the holding time was increased when the temperature is at $675^{circ}C$. The tensile strength and elongation of a reverse transformed steels added with Ti or Nb was 93 kg/$ extrm{mm}^2$ and 40%, respectively. This steel shows higher strength more than 10% of the 0.15C-6Mn steel without loss of ductility. The autenite formed from the reverse transformed treatment has a fine lath type, which has the width size of 0.1-0.3 $mu extrm{m}$. The TRIP sequence normally transforms the austenite to martensite, however, some of the sequence will produce retained austenite longrightarrow deformation twin longrightarrow martensite