Steel moment frame connection with Reduced Beam Section (RBS) is one of several pre-qualified connections which have been proposed in FEMA 350 for use in moment resisting frame structures. Previous studies on behavior of RBS connections are limited to connections with rolled sections and design requirements have been developed for such sections. Large size rolled sections are not readily available in developing countries like Iran and steel frame structures are usually built using plate girders. In such structures the slenderness ratios of web and flanges could greatly influence the seismic performance of the RBS connection. In this paper the effect of slenderness ratios of web and flanges on the behavior of RBS connections is studied by nonlinear finite element analyses. The analyses simulate inelastic local buckling of the girder as ductility and energy dissipating capacity of the connection are directly influenced by such inelastic behavior. Twelve RBS connections with various web and flange slenderness ratios are analyzed to evaluate the effect of slenderness ratios on ductility of the connection. The results indicate that FEMA- 350 requirements for maximum slenderness ratios of web and compression flange are too conservative. Connections in which the slenderness ratios of girder web and flanges exceeded the allowable limits by up to 30 percent have shown proper ductile behavior in the analyses.