This paper deals with the problem of buckling of rectangular plates under uniform compression, which are simply supported along two loading edges and are simply supported but partially clamped along the other two sides. The energy method is used in investigating this problem which was proposed in our previous papers for solving the general problems of statical deflection, stationary vibration, buckling, etc. of rectangular plates. Numerical calculations are carried out on ten cases, and the diagrams are obtained which give the buckling loads for the rectangular plates of the aspect ratio 1, 2/3 and 1/2. These results are checked by experiments.