Tendon Force is an integral value of inherent stress associated with inherent strain, which is the root cause of welding-induced longitudinal residual stress, shrinkage and bending distortion. Also, Tendon Force concept is a very useful tool for estimating the weld longitudinal shrinkage and bending distortion in large welded structures based on the elastic finite element solution. Several predictive formulae for Tendon Force have already been proposed on the basis of welding conditions, but a unified understanding of the effect of welding conditions on Tendon Force has not yet been achieved. In this paper, dominant factors influencing Tendon Force in welded structural materials were discussed through the use of theoretical and numerical approach to quantify the effect of welding conditions, such as weld heat input, material properties and dimensions of weld joint, on Tendon Force due to welding. First, numerical investigations on the effect of welding conditions on Tendon Force were systematically performed using FE-analysis. After that, the numerical results obtained were arranged according to thermal elastic-plastic parameter, which was derived from the theory of thermo-mechanical process in welding. Based on the results, it was clarified that a unified evaluation of the effect of welding conditions on Tendon Force could be achieved by means of the thermal elastic-plastic parameter. The usefulness of thermal elastic-plastic parameter was thus verified as the dominant factor influencing Tendon Force due to welding.