A new Mn-series of oil-quenched super-strength bainitic steel was developed on the basis of the alloy system of the original Mn-series air cooled bainitic steel. The mechanical properties of the new steel are well combined while precious alloy elements and complicated technologies are not needed for its production. Experimental results show that bainite/martensite (B/M) dual phase microstructure can be obtained within a very wide cooling rate range and bainite microstructure exists even when the cooling rate reaches as fast as 60 ℃/s; the hardenability of the steel is better than that of 40CrNiMo and 42CrMo; the diameter of the round bar made of the steel is 250 mm, and the round bar tempered at 350 ℃ after oil quenching exhibits its central mechanical properties as follows: Rm=1550 MPa, Rp0.2=1270 MPa, A=15%, aku=95 J/cm2 (at room temperature). A new Mn-series of oil-quenched super-strength bainitic steel was developed on the basis of the alloy system of the original Mn-series air cooled bainitic steel. The mechanical properties of the new steel are well combined while precious alloy elements and complicated technologies are not needed for its production. Experimental results show that bainite / martensite (B / M) dual phase microstructure could be obtained within a very wide cooling rate range and bainite microstructure even when the cooling rate reaches as fast as 60 ° C / s ; the hardenability of the steel is better than that of 40CrNiMo and 42CrMo; the diameter of the round bar made of the steel is 250 mm, and the round bar tempered at 350 ° C after oil quenching exhibits its central mechanical properties as follows: Rm = 1550 MPa, Rp 0.2 = 1270 MPa, A = 15%, aku = 95 J / cm 2 at room temperature.