Laser beam welding of high quality and high efficiency has been applied to precision parts such as core internal parts in nuclear power plants. In order to apply laser beam welding to thick plate and large scale products, high power laser beam need to be transferred and deep penetration welding procedure must be developed. In this paper the optical fiber transmission system for 7 kW-class high power YAG laser and pulsed laser welding techniques to obtain deep penetration were developed. Furthermore this procedure has been applied to the welding of stainless steel tanks for nuclear fuel reprocessing plants.