In this paper, the compressive strength of long rectangular plates (aspect ratio α=3 and α=4) under increasing compression and constant hydrostatic pressure, is studied theoretically and experimentally as a basic study on the compressive strength of a ship's bottom plating. The theoretical calculations are performed by following two methods assuming that the plate behaves elastically up to the collapse. a) The method assuming that the plate collapses when the normal stress in the direction of compression at the longitudinal edges of the plate will become equal to yield stress of the material. b) The method assuming that the plate collapses when the plate will satisfy the condition of plastic collapse based on plastic analysis in which collapse mechanism is assumed and the large deformation theory is considered. From these results of theoretical calculations and experiments, conclusion is summarized as follows : (1) The compressive strength for comparatively thin plates having the large value of ( b / t ) √σ _Y / E where collapse of the plate will occur in postbuckling state does not change so much with hydrostatic pressure, while for comparatively thick plates having the small value of ( b / t ) √σ _Y / E where collapse of the plate will occur in prebuckling state, the compressive strength changes remarkably with hydrostatic pressure. (2) Calculated values of the compressive strength based on the method a) are generally larger than those based on the method b). The difference of the two becomes more remarkable for thicker plates having the small value of ( b / t ) √σ _Y / E under larger hydrostatic pressure. (3) Experimental results of this study concerning with thin plates having comparatively large values of ( b / t ) √σ _Y / E agree generally with the tendency of theoretical results.