Abstarct: Statistical data from past-experienced earthquakes, such as the hyogoken-Nanbu earthquake that occurred in 1995, shows that falilure of pile foundation happened frequently, so it is necessary to investigate the behavior of pile foundation during strong earthquakes and improve its seismic behavior. One of the most effective and economical methods for improving the seismic performance of existing pile foundations is partial ground improvement. In this method, the main issue is to specify the size and position of the improving area. In this study, the pattern of the partial ground improvment for an existed micropile group foundation is carefully selected using three-dimensional numerical models and elasto-plastic static finite element analysis by acting a lateral pushover force to the foundation. The maximum horizontal displacements of the micropiles in different types of improvements are obtained and then, the optimal pattern according to the concept of effective improvement has been chosen Then, with several dynamic analyzes subjected to two different earthquakes with different frequencies acted on three structures with frequencies equivalent to 5, 10 and 15 story, the effects of the partial improvement on the seismic performance of the foundation, such as maximum shear forces, maximum axial force and bending moment created in the micropile have been evaluated. In this research, the high pressure injection (jet grouting) method has been selected to improve the foundation system, which provides the highest strength and hardness among all existing methods for soil improvement. The results of the analysis showed that partial improvement significantly reduces internal forces such as shear force, axial force and bending moment, which increases the lateral strength of foundation. Also, this improvement has significantly reduced the recorded acceleration on the foundation and structure, which improves the seismic performance of the micropile group- foundation.