An analytical formulation has been developed for the electromagnetic leakage from an apertured rectangular cavity excited internally by an electric dipole. The formulation consists of three parts: the interior problem of determining the field distribution within a closed cavity with the eigen-mode expansion method; the problem of characterizing the leakage mechanism of aperture by using the Bethe’s small hole coupling theory; the exterior problem of determining the transmission field by using antenna theory. Theoretical values of shielding effectiveness are in good agreement with full-wave simulations. Analytic approaches, disregarding limited scope of application, are still of realistic interest in both science and engineering aspects due to clear physical meaning, high computation efficiency and easy to be programmed. In this paper, an analytic formulation is implemented to investigate the electromagnetic leakage from an aperture rectangular cavity. The formulation is based on the Bethe’s small aperture coupling theory, which uses equivalent electric and magnetic dipoles to describe the relationship between the internal field of a closed cavity excited by an internal source and the leakage field from apertures. This Bethe’s theory had been employed by some authors in earlier papers, and it is shown that the approach can work very well provided that the size of aperture is much smaller than one wavelength.