For constructing the steel structures of ship and automobiles etc, thin plate welding has been extensively used in recent years. However. the welding of plate produces welding deformation and residual stress which sometimes extremely harm to the safty of structure in the course of construction and operation. Therefore, in order to accurately prediet the welding deformation and residual stress, it is important to exactly analyze the distribution of temperature during welding in thin plate and take into account the moving effect of the heat source. In this paper, two dimensional unstationary heat conduction problemes of thin plate are formulated using an isoparametric finite element. After the development of the computer program, this method is applied to some specimens, and the analyzed results are compared with the experimental ones to confirm the usefulness of this method.