ABSTRACT: Rectangular steel plates are commonly used in the aviation and ship industries, as well as in steel structures because most engineering facilities are looking for a lightweight structure that has high resistance at the same time. Therefore, the problem was how to strengthen the rectangular steel plate bound on four sides along its borders without increasing the size or weight of the plate after strengthening it. This is the main goal of the thesis, as this thesis examines the elastic post-buckling behavior of a rectangular steel plate constrained from four sides when the plate had embedded stiffener by making a zigzag inside the constrained plate at the expense of its thickness, meaning that the thickness of the plate decreases after it is included stiffener and its length increases by a small percentage so that a weight of the plate, after stiffened, is equal to the weight of the plate before stiffened. The parameters investigated in this thesis are divided into two parts. The first part: Parameters specific to the implicit reinforcement: the shape, height, and location of the implicit reinforcement. Where the shape is (a rectangle, a triangle, or a circle), the height is (1t, 2t, 3t), and the location is (in the middle, a third, or a quarter of the length of the plate). The second part is parameters specific to the steel plate, which are the aspect ratio (1, 1.5, 2, and 3), and the slenderness ratio (100, 80, 60, and 40). The nonlinear analysis of the elastic post-buckling behavior of 216 samples was conducted using the Abacus program, version 2020, using the RIKS method. The focus of this analysis was on the geometric nonlinearity only for the steel plates reinforced with implicit reinforcement. The analysis is divided into two parts according to the direction of the compressive load concerning the implicit reinforcement. Part one: 108 samples when the compressive force is perpendicular to the direction of the implicit reinforcement. The second part: 108 samples when the compressive forces are parallel to the direction of the implicit reinforcement. It was found that when the direction of the compressive forces is perpendicular to the direction of the implicit reinforcement, and the shape of the reinforcement is rectangular, its location is in the middle of the plate, and its height is equivalent to three times the thickness of the plate, deformation is reduced by a greater percentage than other options. The analysis was compared with a previous study analyzing the post-buckling resistance of a steel plate of rectangular shape, constrained along its four edges.