Plastic design methods assume that local buckling of flanges and webs of menbers will not occure during the formation of plastic hinges. In view of such a fact, plate buckling in the plastic range becomes a foundamental problem to determine the dimensions of the members. In this paper, so-called “Δ²-method” which gives experimentally the critical value of elastic plate buckling is extended to plastic plate buckling problem, and a new method (ε-Δ² method) is introduced in order to obtain the experimental buckling load of plastic buckling of the uniformly compressed plate. This new method bases on the linear relation (the average compressive strain) and Δ² (the deflection of plate) after plate buckling. Moreover, the compression tests of fifteen short angles with various b/t were carried out. The critical values experimentally decided by ε-Δ² method, were compared with the theoretical values. As a result of the comparison, the following were obtained conclusively. (1) From the theoretical calculation, the average compressive strain of uniformly compressed plate into plastic range are approximately linearly related with the square of the deflection Δ² after buckling. (2) The experimental values of the angle tests clarified such a matter that was mentioned in (1). (3) The experimental values of the angles coincided well with the theoritical values. (4) The value of b / t necessary to prevent plastic plate buckling until the strain reaches to the strain hardening zone, was about 67 for 70 (kg/mm²) high tensile steel and about 7.5 for mild steel, and generally the higher the yield stress, the smaller the required value of b / t .