As a new construction material, fiber reinforced polymeric plastic structural shapes are readily available. Therefore, new construction and existing structure rehabilitation using FRP materials are ever increasing trend because of FRP material’s chemical and mechanical properties compared with those of conventional construction materials such as steel and concrete. Among the structural composites, pultruded fiber reinforced polymeric plastic structural members are the most popular for the civil engineering applications because of the cost effectiveness. However, they are made of fiber reinforced polymer resin system, they have relatively low modulus of elasticity and also cross-sections of structural shapes are composed of thin plate components such as flange and web. Therefore, structural stability is an important issue in the design of pultruded structural members. For the design of pultruded structural member under compression, buckling and post-buckling strengths of plate components may be taken into account. In the structural steel design following AISC/LRFD, in addition to the buckling strength, the nonuniform stress distribution in the section is incorporated with a form factor. In this paper, the form factor for the design of pultruded structural member under compression is investigated by experiment. Based on the experimental results, the form factor for the design of pultruded structural shapes have been suggested.