The objective of this study is to find the effect of ultrafast cooling on microstructure and mechanical properties of steel with different carbon composition. Steel samples of 6 mm thick AISI 1008, AISI 1040, AISI 1075, and AISI 1090 have been used in the ultrafast cooling experiments to study the effect of carbon percentage on the cooling rate and mechanical properties. Air-atomized spray cooling system has been deployed with a fixed air flow rate and varying water flow rates. Experimental results have shown that, as water flow rate is increased, the cooling rate and surface heat flux increase for all grades of steel. It has been observed that the steel with higher carbon content requires higher water flow rates to reach an ultra fast cooling rate of more than 140°C s−1. After cooling experiments the microstructure, tensile strength and hardness of the samples have been examined and it is found that increase in carbon content affects the cooling rate and also consequently the microstructure. The tensile strength and the hardness of the final product after the ultrafast cooling process conform the advanced high strength steel (AHSS) grade with high martensite phase composition of 65% to 95%.