The low－alloyed transformation induced plasticity (TRIP) steels demonstrate an improved combination of strength and ductility, and have became a promising candidate for the application of automotive bodies to reduce the weight without the loss of crash－worthiness. The typical microstructure of TRIP steels consists of the ferrite matrix and a dispersion of bainite, martensite and the retained austenite. The existence of an amount of metastable retained austenite is responsible for the improved mechanical properties, resulted from the enhanced strain hardening capabilities of TRIP steels due to the strain－induced martensitic transformation during straining. The carbon content is considered as an important factor that influences the amount and stability of the retained austenite. In the present work, two cold－rolled C－Mn－Al－Si TRIP steels with different carbon contents, (0.1% and 0.2%, mass fraction) were fabricated by intercritical annealing and isothermal transformation. The microstructures and the mechanical behaviors of the used steels were investigated by OM, XRD and uniaxial tensile tests at room temperature. The results indicated that with the same isothermal transformation time at 400℃,the steels with high carbon content obtained lower fraction of bainite and larger fraction of martensite, and demonstrated higher strength and larger elongation than those of steels with low carbon content. The excellent ductility of steels with high carbon content was mainly attributed to its strong TRIP effect during deformation, resulted from the larger fraction of retained austenite as well as the higher carbon content of retained asutenite in the multiphase microstructure. The value of the product of tensile strength and total elongation, representing the combination of strength and ductility of steels, was increased linearly with the increase of the value of the product of volume fraction and carbon content of retained austenite, which could be used to characterize the TRIP effect. Variation of the formation rate of strain－induced martensite was similar to that of the incremental strain hardening exponent with strain during deformation, further proved the important role of TRIP effect in influencing the strain hardening capabilities of TRIP steels.