In order to reduce the cost of alloying elements, low carbon Mn series bainitic steels have been developed. The effect of tempering temperature on the microstructure and low temperature impact toughness of a low carbon Mn-series steel has been investigated in the present study. The as rolled steel plate samples with 30 mm thickness were tempered from 280 ℃ to 600 ℃ for 2 h. Metallographic microstructure show that the< microstructure of the as-rolled steel is mostly bainite laths. Bainite laths start to merge and broaden after tempering at 460 ℃, and quasi--polygonal ferrite structures could be revealed after tempering at 600 ℃. Compared with the as-rolled steel, after tempering at 460 ℃, the yield strength of the steel changes slightly, remaining 725 MPa, while the Charpy absorbed energy at $-$40 ℃ increases from 38 J to 146 J, and the ductile-brittle transition temperature (DBTT) decreases from -18 ℃ to -48 ℃. The EBSD and TEM results indicate that the improvement of low temperature impact toughness after tempering at 460 ℃ is caused by the increase of fraction of high angle boundaries and the decrease of effective grain size during the recovery process of bainite laths.