In this paper, the influence of plate thickness on the fatigue strength of non-load-carrying fillet weld joints is discussed, from the viewpoint of the residual stress and the stress concentration. The calculation methods of the weld residual stresses in cruciform joints and T joints are presented, and the residual stresses are expressed in functions of plate thickness, yield strength, and heat input. The calculated residual stresses are compared with the experimental ones, and are in good agreement with. The combined effects of mean stresses (or of the stress ratio) and the weld residual stresses are applied to the fatigue strengthes of the cruciform joints and the T joints, and it is confirmed that the difference in the fatigue strength between the two types of joint is due to the difference of the residual stress distributions. In the last part of the paper, the estimation method on the fatigue strength of the non-load-carrying fillet weld joint is presented. By means of the method, the fatigue strength of the joint is estimated, with the fatigue strength of the base metal, the stress ratio, the stress concentration due to the joint, and the residual stress. On 67 sets of the test results, the applicability of the estimation method is confirmed.