Conventional electric field integral equation-based theory is susceptible to the spurious internal resonance problem when the characteristic modes (CMs) of closed perfectly conducting objects are computed iteratively. In this paper, we present a combined field integral equation-based theory to remove the difficulty of internal resonances in CMs analysis. The electric and magnetic field integral operators are shown to share a common set of nontrivial characteristic pairs (values and modes), leading to a generalized eigenvalue problem which is immune to the internal resonance corruption. Numerical results are presented to validate the proposed formulation. This work may offer efficient solutions to CM analysis which involves electrically large closed surfaces.