Abstract: To achieve the objective of both high strength and plasticity in TRIP (transformation induced plasticity) steels, the influences of alloying elements and isothermal treatment temperatures on mechanical properties were investigated by tensile test, SEM, TEM and XRD techniques. The results indicated that with the isothermal treatment temperature increasing, the yield strength and the tensile strength decrease initially then increase. Nevertheless, the elongation and the combination of strength and elongation follow the countrary trend of the strength. The maximum tensile strength of over 1000 MPa and combination of strength and elongation of 22080 MPa·% are achieved when the bainite isothermal temperature is below 320°C or above 480°C. When the isothermal treatment temperature is within the bainite region of 400°C, the highest elongation and combination of strength and elongation are 31% and 27150 MPa·%, respectively. If Si is substituted partially by Al, the tensile strength decreases. As 0.5% copper is added, the tensile strength increases while the total elongation value decreases. The combination of strength and elongation under such a condition is 25085 MPa·%. Thus, under certain heat treatment process, high tensile strength and elongation of steels containing proper alloying elements can be obtained.