The authors investigate the main strength characteristics of the 50kgf/mm² class newly developed controlled rolling steel plates especially for the welded joint made by high heat-input welding and also evaluate the fracture toughness of these plates, when they are used for ship's hull. And also investigate the crack arresting properties of the newly developed controlled rolling steel plates. That is, this report shows the fracture toughness of Z direction (at the middle of plate thickness) and of the welded joint by high heat-input welding, and also proposes a method of estimating the values of fracture toughness of welded joint from V-notch Charpy absorbed energy. When these plates are used as the material of ship's hull, it is possible to adopt the welding method of the highest class heat-input welding that has been used in the building stage of shipbuilding yard. And after comparing K _ca characteristic between these plates and conventional process steel plates (Normalized or Quenched and Tempered), a possibility of using these newly developed controlled roiling steel plates as “crack arrester” for brittle fracture is discussed, supposing that necessary K _ca value for “crack arrester” should be greater than 600 kgf√mm/mm² ( K _ca≥600 kgf, √mm/mm²).