Abstract: In order to explain some different views about principle of causing and relieving welding residual stress, a steel bar with two constrained ends and a long welded plate are analyzed as one-dimensional models. Two thennal cycles are used. One includes heating and cooling stages and the other includes only cooling stage directly from high temperature to room temperature. The results show that in the -welds the residual compressive plastic strains are caused in the former case and the residual contractive thermal strains are caused in the second case. Their actions are equivalent for causing the welding residual stress. However it is the best way to introduce the concept of inherent strain which includes the thermal strain,plastic strain and phase transformation strain caused in welding cycle and can be considered as a source of welding stress. Some relative problems are dbcussed in this paper.