In recent time there are vigorous requirements for the use of thick steel plate in various industrial fields, including shipbuilding industry. Especially, application of TMCP steel plates on steel structure is progressively increasing now. Welding process with high heat input is necessary to be used in thick steel assembly for the purpose of the high productivity. However, HAZ softening of TMCP steel plates has a possibility to reduce the strength of welded joint. In this study, therefore, the tensile strength of TMCP weldments, having softened HAZ, was examined using the numerical calculation and experiments.