In this research, the behavior of steel structures including the characteristics of semi-rigid connections was analyzed by computer simulation under artificially generated earthquake loading since the experimental study for the whole structures is alomost impossible in the current situation. From the results of analyses for the 1, 2 and 3-story structural models dealt in this research, the lateral rigidity of steel structures with characteristics of semi-rigid connection was less than that with characteristics of rigid connections in the elastic zone. On the other hand, the ductility factor in the case of semi-rigid connections increased. The ductility factor is important for accuracy of earthquake resistance design by the equivalent static analysis and the response spectrum analysis. Therefore, these facts can be explained by that the classical method supposing semi-rigid connections as rigid body for the elastic design of steel structurs could be unstable. And over-estimate design in the case of semi-rigid connections is expected by higher ductility factors and by decreasing of magnitude of response spectrum due to the increment of natural period. The result of this research shows that there is difference between the behaviors of steel structures considering semi-rigid and rigid connections. Therefore, it can be concluded that the characteristics of the semi-rigid connections are very meaningful by increasing necessity of more accurate analysis of structures.