Abstract: Low carbon Mn-Mo-Nb-Cu-Zr-B steel was designed, which was processed by thermo-mechanical controlling and cooled at medium cooling rate, and the microstructures including an abundant of lath bainite, a number of granular bainite and a bit of acicular ferrite were obtained. The yield strength of the as-rolled steel plate exceeds the 850MPa grade, which meets the need for the mechanical properties of X120 pipe line steel. As 0. 015% Zr (mass faction) was added in the steel, a number of complex Zr-bearing carbonitrides with irregular shape were observed by TEM, and their size ranged from 80 nm to 200 nm; considering anomalous shape, they formed at high temperature and had high melting points. The size of Zr-bearing precipitates would reduce when the reheat temperature exceeded 1200℃ because of some amounts of Ti and Nb in the precipitates dissolving, which is better for controlling the austenite grain coarsening. Whereas, quasi-elliptic Zr-free carbonitrides would dissolve even vanish when reheating. Zr-bearing precipitates distributing evenly can block the moving of grain boundaries effectively when reheating, which is the predominant reason that the tendency of austenite grain growth of Zr-bearing steel is lower than that of Zr-free steel. A conclusion should be obtained that 0.015% Zr (mass fraction) addition can improve the weld properties of low carbon Mn-Mo-Nb-Cu-Zr-B steel.