Abstract: In the process of developing ULC SM490 A by Ti microalloying,excessive strength fluctuation was caused by different Mn content. Therefore,the influence of Mn on the microstructure and mechanical properties of ULC Ti microalloy steel has been studied. It is found out that when the Mn content was increased from 0.4% to 0.8% for base chemistry of 0.07%C+0.06%Ti compositions,the mechanical properties of the steel were significantly improved,the yield strength was increased by around 161 MPa,the tensile strength was increased by around 141 MPa,and the elongation after fracture decreased slightly.As the Mn content is increased,the microstructure of the steel has not changed,composed of ferrite and pearlite,but the ferrite grain size number is increased from 9.5 to 11～11.5,and the corresponding strength increment is about 44 MPa and 61 MPa calculated by the Hall-Petch formula.The total strength increment of about 35 MPa by those of Mn decreasing transition point and cause an increase in the dislocation density,plus [Si] and [Mn] substitute solution strengthening.Then it can be assumed that the strength increment due to TiC second phase strengthening by Wagner interaction of Mn element was about 65 to 82 MPa.TEM observation showed that when Mn mass fraction was increased,the TiC volume fraction was significantly improved about 3 times and particle size was about 10 nm,which was coincided with the second phase strengthening calculation.Therefore,it can be inferred that in the actual production process,the strength control of microalloyed steel can be achieved by adjusting Mn content,which undoubtedly has a positive effect on the development of microalloy steel.