Recently laser power is increased rapidly and the laser welding ability is advanced, then we can use it for thick plate welding in heavy industries. In such a background, we aimed to apply YAG laser welding to manufacture our products, and introduced 10kW and 7kW class YAG laser processing system. Then we have developed the optical fiber transmitting technology for high power beam, and welding technology which could make over 20mmt 1pass weld joint. To achieve such welding ability, we optimized welding parameters and controlled keyhole state, penetration shape and welding efficiency. Then we could get the high quality weld efficiently. After we confirmed the mechanical property of weld joint based on the examination standard for power plants, we applied the YAG laser welding to manufacture stainless vessels for reprocessing plants.