The study presents the bending solutions of thick rectangular plates with clamped at three edges and simply supported at the remaining one edge (CCCS) carrying a uniformly distributed load applying energy method. Higher order shear deformation plate theory was used to formulate the governing differential equation by applying the principles of elasticity. Total potential energy equation of a thick plate was formulated from the constitutive relations thereafter the three general governing differential equations for the determination of the out of plane displacement and shear deformations rotation along the direction of x and y coordinates were obtained. The total potential energy was in the same way used by the method of direct variation to obtain three simultaneous direct governing equations for the determination of deflection and shear deformations coefficients. From the formulated relationship, formulars for calculation of the inplane and out plane displacement shear deformation rotations along x and y axis, stresses, moments and stress-resultants expressions were deduced. Unlike First order shear deformation theory (FSDT), this higher order shear deformation theory as was proposed does not require shear correction factor in the analysis. It can be concluded that the rectangular plate can be classified as thick plate, when the plate span-to-depth ratios is less or equal to twenty (/≤); it can be classified as moderately thick plate, when the plate span-to-depth ratios is between twenty and fifty:(≤/≤); when the plate span-to-depth ratios is greater or equal to twenty plate:/≥. This assertion can be used to shows the boundary between thin and thick plate. Furthermore, it is seen that at the 96% confidence level, the values from the present study are the same with those from of previous studies, confirming the accuracy and reliability of the derived relationships.