The application of plates becoming widespread in various branches of engineering as main structural elements, the strengthening of them in contrast with environmental loads becomes important. Among different environmental loads, inplane loads are important which cause buckling in plates. So, decreasing the hazard of buckling in plates is one of the interests among designers and researchers. A way to increase the strength of plate is to increase the whole thickness of it. Using thick plate especially while the plate is considerably high needs much more material and the weight becomes enormous .One of the ways to decrease the cost and the weight of plate structure is to increase the thickness locally. Increasing the thickness of the plate locally gives an efficient cost structure with enough buckling strength but not as much as thick plate. In order to supply required strength in such plates, it is suggested to add longitudinal stiffeners to them. Interest in stiffened plate construction has been propagated during recent years due to it's economic and structural benefits. Although the stiffening elements cause a negligible increscent in the weight of overall structure, their influence on strength and stability is enormous. In this case a part of required strength would be provided by locally increased thickness of the plate and the remained would be provided by added stiffeners. This means that, using this method neither needs increasing too much the thickness of plate, nor using stiffeners. Then achieved structures would be cost efficiently with proportional weight. This study is concerned with buckling analysis of stiffened plate with step variation in thickness in either one or two directions. In addition, sometimes the critical stress exceeds proportional limit of the material. So the plate structure goes to the plastic range. In this case buckling problems become very important since the stiffness matrix of the structure is not constant, either. By using the theory of plasticity and stress-strain curve inelastic buckling problem can be solved which is discussed in this thesis. However, adding stiffeners to the plate in addition to locally increasing thickness of the plate complicates the analysis of such plates. Therefore, due to complexity of the closed form solutions for buckling analysis of such plates, numerical methods are more recommended. Among the various numerical methods, finite strip is an efficient method for prediction buckling stress of plate structures. Comparing with the conventional finite strip method, spline finite strip is more accurate and flexible. In this method, piecewise cubic polynomials which is called B-Spline functions are used as the longitudinal displacement functions. Measuring of abrupt changes in thickness or loading of plates and various boundary conditions becomes possible by amending the B3-spline functions. Different numerical examples in the concept of elastic and inelastic buckling are considered to show the accuracy and versatility of the proposed method. Keywords: Spline Finite Strip, Stepped plates, Stiffened Stepped plates, Locally increased thickness