Abstract: Based on the austempering process, the effects of overaging temperature on the microstructure evolution and mechanical properties of low carbon micro-alloyed steel with aluminum addition were studied. The results show that the microstructure of test steel with different processes condition consists of intercritical ferrite, martensite, bainite and retained austenite.With the increase of overaging temperature, the fraction of martensite decreases and the grain size of M/A island is refined, resulting in the decrease of strength and significant improvement of total elongation.Based on the result of XRD, the amount of retained austenite is up to 8.2%, when overaging temperature is 380 ℃. Morever, it can be found that the instantaneous work hardening exponent curves of specimen with relatively higher overaging temperature show continuous TRIP effect, which is beneficial to obtain a good combination of strength and ductility. The excellent mechanical properties with tensile strength of 787 MPa, total elongation up to 38.9%, and product of strength and elongation(PSE)above 30 GPa·%is obtained after overaging at 380 ℃.