On the majority of motorcycles, the body structure is a framed construction. In the process of structural design for the motorcycle frame, a numerical optimization assisted by computer is vital as effective measure to determine the best design specifications all the time. Conventionally, however, in nearly all of the examples in which sensitivity analysis and optimum design methods were applied to frame structure, the design variables were limited to the cross-sectional dimensions of the members. In the study presented here, a minimum mass design of the motorcycle frame was aimed in which the cross-sectional dimensions of members and even the positions of members could be employed as design variables. As a result, rather than merely changing the cross-sectional dimensions, it was possible, through structural optimization accompanying changes in the shape of the body, to obtain frame structure specifications in which the mass of the body was reduced greatly while retaining the same level of rigidity.