A method of substructure mode synthesis for determining the dynamic characteristics of an undamped system in a specified frequency band is investigated. The motion of each substructure is represented by the three mode sets: inertia, selected normal, and residual modes. Inertia and residual mode sets are introduced to approximate the truncated lower and higher normal modes. Based on energy considerations, various combinations of modal approximations of each substructure can be used. By applying the zero-eigenvalue coupling technique to interface compatibility and equilibrium, an independent set of system equations is derived. Numerical results indicate that the proposed synthesis method is effective.