The vehicle body stiffness is one of basic and significant design factors because it affects several vehicle performances - dynamics, safety, durability and so on. A sophisticated consideration is needed whether the current evaluation method is adequate for an efficient design of vehicle body structure. In this paper, an analytic evaluation method of the vehicle body stiffness is proposed. The inertia relief method is utilized to minimize the effect of fixed boundary conditions. In order to verify the method, two parametric studies are performed and the results are compared with ones of the current evaluation method. It is demonstrated that the proposed method provides the opportunity to efficiently express and understand the behavior of the complete vehicle.