Recently, it has been very active to develop various types of marine structure and ice breaking vessels in order to exploit energy resources in ice fields. These steel structures are to be operated in severe environments of low temperature in the arctic area. Expecially, ice breaking vessels are required to navigate smoothly with breaking ice floe, thus the side shell and the deck structure should be fabricated of heavy plating with a good notch toughness. To obtain excellent notch toughness and mechanical properties, the authors introduced the new steel manufacturing process, namely CLC process and have successfully developed 50 kgf/mm² class high tensile strength steel of heavy thickness. Then, experiments associated with fabrication, welding procedure and joint performance were carried out for this newly developed heavy section steel to study its applicability to hull structures of ice breaking vessel. It was consequently made clear that this steel could be welded by conventional welding procedure and showed a satisfactory performance which could assure the safety to be required for ice breaking vessel.