Abstract: Effect of rare earth elements such as La and Ce on microstructure and weldability of Q355B steel were investigated by optical microscope,scanning electron microscope,tensile tester and impact tester. The results showed that the Q355B steel to which the trace La and Ce elements were added exhibited microstructure consisting of ferrite and pearlite,no banded structure in the rolling direction,significantly refined grains,and improved microstructure uniformity. In addition,the large-sized elongated MnS inclusions were modified into small-sized spherical rare earth oxysulfide composite inclusions in the steel,the hardness and tensile properties of the welded joint were significantly improved,the grains in the heat-affected zone were refined,and the impact toughness was obviously enhanced.