This study deals with the effect of rotational stiffness of bushings on the substructure coupling by frequency response functions (FRFs). In particular, a simple FRF coupling model is investigated to grasp the trend of error in predicting the natural frequencies of the coupled structure that has zeros in rotational stiffness of the coupling element matrix representing bushings. It is understood from this study that the natural frequencies of the coupled structure are downshifted and the level of response is not obtained correctly without reflecting the rotational stiffness of coupling element matrix. In practice, however, the rotational stiffness of bushings is seldom included in vehicle application because of the considerable experimental effort and time to obtain them. In order to identify the necessity of including the rotational stiffness of bushings in vehicle application, an FRF coupling model is constructed with the vehicle body and front axle. The results obtained from this model reveal that excluding the rotational stiffness of bushings produces a downshift in the natural frequencies by 11 Hz at maximum, indicating the necessity of reflecting the rotational stiffness of bushings in vehicle application.