Seismic performance evaluation of existing structures and retrofit strategies are considered as a complicated multi-criteria problem. This study presents a performance evaluation of a gravity load designed or designed to earlier codes six stories asymmetrical flat slab building. Building with flat slab system may lack the required lateral stiffness to withstand seismic lateral loads. Three different retrofit systems to increase the lateral strength and stiffness of the building are proposed and examined. These systems include the addition of external frames, using shear walls and introducing steel knee bracing. The 3-D nonlinear pushover analysis procedure is utilized in evaluating the seismic performance of the original building and the retrofitted one, capacity and demand spectrum curves are presented for each studied case. The results are compared with elastic time history analysis method. Three different real earthquake records with intensities suitable for zones I, III and IV appeared in the renewed (2003) version of the Egyptian code of loads are utilized. It is found that the applied 3-D nonlinear analysis can be considered as a powerful tool in evaluating the seismic performance of frame structures. The study also shows that how can every suggested retrofitting system upgrade the seismic behavior of the gravity load designed flat slab building. Cost analysis comparison study is carried out.