In this paper, the behaviour and the compressive strength of long rectangular plates (aspect ratio α=3 and 4) with four clamped edges under increasing compression and constant hydrostatic pressure, are studied theoretically and experimentally following the previous papers. Results obtained in this study are summarized as follows : 1) The basic equations for pursuing the elastic behaviour of the plate with four clamped edges under such combined loads, are led by means of the energy method. Carrying out the analyses based on these equations, characteristics of behaviours including buckling phenomena are clarified. 2) The compressive strength of such a plate is calculated in the same way as the previous paper, and characteristics on the compressive strength and the collapse mode of the plate with four clamped edges are clarified. From these results, it is found that the tendency of the variation of the compressive strength due to hydrostatic pressure is generally similar to that obtained for plates with simply supported edges. 3) Performing tests for thin plates (α =3) having comparatively large value of ( b / t ) √σ _Y / E , the obtained results on behaviours including buckling phenomena and compressive strength are in good agreement with theoretical results.