800 MPa grade new ultra-low carbon bainitic (NULCB) steel was welded by laser welding and melt-active gas (MAG) welding, and the microstructure and mechanical properties of the heat-affected zone (HAZ) were investigated. The experimental results indicate that the microstructure of the HAZ is only granular bainite which consists of the bainite lath and the martensite-austenite (M-A) constituent. As the heat input that was adopted in the experiment increases, the average width, gross and shape parameter of M-A increase, but the line density decreases. The toughness of HAZ is higher than base metal under appropriate laser welding conditions. With the heat input of laser welding increase, the hardness of welded joint decreases, but it is higher than that of the base metal, indicating no softened zone appeared after laser welding.