Tensile tests are carried out by using the welded plate with two parallel cracks. The effects of the residual stress, the hardened zone by welding and the interaction between cracks on the plastic deformation near cracks and the crack opening displacement are examined by the experiment as well as the elasto plastic fi­nite element analysis. A new technique for evaluating nondestructively the plastic deformation near cracks by using the infrared thermography is proposed. The heated region and the temperature rise measured by the thermography are compared with the plastic zone and the work of plastic deformation obtained by the analysis. The validity and the limitations of the application of the infrared thermography to the evaluation of the plastic deformation are discussed.