YAG laser welding for body in white has been developed and applied to the joint of the roof and body side of new L-class car. By comparison with the resistance spot welding, which is conventional joining method, the laser welding has some advantages, such as one-side welding and continues welding. So the joint structure can be improved to lightweight with high strength and stiffness. And the appearance of joint also is improved.We use two 3KW YAG lasers and robots with fiber optics. The roof and body side panel are galvanized zinc coated steel, and lap welded. The most critical factor for mass production is the gap control technique, and we developed the gap control system with special pressure device. And in-process monitoring system is also developed which analyzed the intensity of plume and back reflection of YAG laser.