Abstract: The effects of tempering temperature on the microstructure and mechanical properties of 600 MPa grade low carbon bainitic steel belonged to Fe-Mn-Nb-B series were investigated by utilizing scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that tempering temperature has considerable influence on both the yield strength and the tensile strength. The yield strength increases with increasing tempering temperature while the tensile strength has an adverse law. In comparison with the as-received steel, the yield strength of the steel tempered at 600℃ increases 105 MPa. With the increase in tempering temperature, the yield strength increase first and has a slight decline after it reaches the highest point at 600℃, the tensile strength decreases significantly, the elongation increases slightly and the yield ratio increases also. Through analysis it is concluded that the major change of mechanical properties after tempering has a connection with dispersive precipitates with the size of less than 20 nm, the decomposition of large M/A islands in which martensite has a large proportion and the reversion of dislocations and polygon ferrite.