In this study 3- and 6-story flat plate structures designed only for gravity load are retrofitted with steel plates and braces and their seismic performances are evaluated to verify the effect of seismic retrofit. According to the analysis results obtained from nonlinear static and dynamic analyses both the strength and stiffness are significantly enhanced as a result of the seismic retrofit. Especially the effect of column jacketing could be enhanced significantly when slabs were reinforced to prevent premature punching shear failure. When buckling-restrained braces are used instead of conventional braces, the structures showed more ductile behavior, especially in the 3-story structure.