The coaxial aperture structure has been under intensive study in recent years, particularly since it exhibits electromagnetic transmission resonances that are stronger than its circular aperture counterpart. In our work, we study the resonance properties of a coaxial aperture in a perfect electric conductor (PEC) and in a real metal. For PEC, The dielectric constant is in finite and for real metal the dielectric constant is fi nite. We develop theory for reflection phase and amplitude in coaxial aperture at the end of a metal plate. While most of the past works of coaxial aperture focused on the propagation of light within the aperture structure and ignore the reflection at end-face,we fi nd that the reflection properties at the end-face are critical to determine both the wavelength and quality of Fabry-Perot resonant transmission of coaxial structure. Finite-di fference time-domain calculations agree well with our theory. We fi rst consider the PEC case, and later to develop the theory to account for real metal case. In real metal, the phase and amplitude of reflection are quantitatively diff erent from PEC because of plasmonic e ffects. Such di fference arises from the new physics associated with surface plasmons. This work is of interest to ongoing studies of coaxial structures in metal fi lms, which could impact many fields including filter e ffect,optical sensing, optical trapping, near- field spectroscopy and metamaterials. / Graduate