Repair welding produces welding deformation, residual stresses, etc. which may newly produce repair welding cracks. Therefore, it is important to clarify the mechanical characteristics of repair welds in thick plates. In this paper, three-dimensional unstationary thermal conduction analysis and three-dimensional thermal elasto-plastic analysis are performed. Based on the results, study is made on the characteristics of the distributions of three-dimensional welding residual stresses and plastic strains, and their production mechanisms. The main conclusions can be summarized as follows: 1 ) Residual stresses and plastic strains in the base plate are largely affected by compressive mean normal stresses and compressive plastic strains produced in the heating stage, respectively. This is remarkable in the base plate near HAZ. 2) Irrespective of elastic or plastic state, severity of mechanical restraints in different directions can be known from the magnitude of produced stress components, {σ_x, σ_y, σ_z}^T. 3) Since plastic strains are assumed to be incompressive, the sign of plastic strain produced in the direction where maximum stress is produced (mechanical restraint is the severest) is opposite to that in the direction where minimum stress is produced (mechanical restraint is the weakest). Therefore, the signs of stress and plastic strain may not agree. 4) For evaluation of the mechanical damage of the material caused by welding, it is necessary to pay attention to the whole elasto-plastic behaviors, in addition to the magnitude of residual plastic strains. In this paper, the plastic work and the sum of the equivalent plastic strain increments are proposed as measures to evaluate the severity of mechanical damage throughout the entire history of thermal elasto-plastic behavior. Both measures become maximum in the base plate near HAZ.