Abstract: The effect of chemical compositions and cooling processes on microstructure and mechanical properties of low carbon and manganese containing niobium and titanium micro-alloyed advanced high strength low alloy steels for automobile were studied in laboratory.The microstructure changed from ferrite plus pearlite to bainite and martensite,and the strength increased when the elongation decreased with the decreasing of coiling temperature.The strength increment of the sheet coiled at high temperature is much higher than that coiled at lower temperature due to the contribution of Nb and V,while the function of carbon went to the opposite.The two different strength class steels S550MC and S700MC can be manufactured by different cooling processes with steel of the same chemical composition about 0.06%-0.11% carbon,and also manganese,niobium and titanium added.