In the design process of an automobile part, several analysis methods are usually used to evaluate the performance of the part. However, most automobile design engineers do not directly use CAE (computer aided engineering) tools since specific skills are required to obtain practical results. Moreover, CAE requires a huge amount of computation time and cost. To resolve these problems, a new design approach, termed first order analysis (FOA), has been proposed. In this paper, the FOA technique is employed to design a vehicle sub-frame. An equivalent model of the vehicle sub-frame which only consists of beam elements is proposed and the modal properties obtained with the model are compared to those obtained with a full scale finite element model. The effects of some parameter variations on the modal characteristics of the vehicle sub-frame are investigated by employing the FOA equivalent model.