Recently usage of composite beams becomes very common. Researchers about buckling of this beam in negative moment have been an interesting subject for many studies. Many methods have been extended to prevent thin web’s buckling. In this thesis a new method is used to stiffen composite beams. To achieve this goal thickness of bottom flange during its width is assumed to be variable. Main process can be divided in to major sections, first studing about local buckling and then distortional buckling of composite beams. For this purpose, at first differential equation of plate is extended, then finite stripe method for analysis of local buckling of composite beams. A simple method is used to assessment distortional buckling of composite beams. Our results have been compared with many references. We used computer program for analysis of local and distortional buckling of composite beams in many engineering problems. Finally we have drawn figures to show our results, which can be used in design of I-section composite beams. According our results the section’s capacity is increased during local buckling, but distortional buckling is not efficient by using this method.