Site effect is known as one of the important issues in geotechnical earthquake engineering. The site effect can change the characteristics of seismic waves and amplify the vibrations which results in the casualties and financial damages. Nonlinear method is an appropriate numerical method for site effect analysis due to its accuracy and its close results compared to the actual soil behavior. Thus, in this research, nonlinear analysis was applied for evaluation of site effect and for achievement of a suitable design spectrum in Babol city located in the north of Iran. For this purpose, first, seismic, geophysical and geotechnical data of Babol city were provided. Then, by modeling the subsurface conditions, the tripartite response spectra were found for different areas of Babol. According to the obtained results, the behaviors of tripartite response spectra are fundamentally different in some frequency ranges. In addition, the shape factors obtained from site analysis for different parts of Babol city were compared with the shape factor of the design spectrum presented by Iranian seismic code. This comparison indicated that the response spectrum obtained through analysis differs from the Iranian seismic code design spectrum. Finally, it can be concluded that the structures designed according to Iranian seismic code are neither safe nor economic in some areas. This issue shows the necessity for more serious consideration of site effect phenomenon in Iranian seismic code.