In this paper, the accurate characterization of the input impedance of a rectangular waveguide with a conducting post and the other discontinuity is presented using mode matching method and the equivalent transmission line theory. By the proposed method, the input impedance of rectangular waveguide with a conducting post and the other discontinuity as a function of the radius and height of the post, the reflection coefficient of the other discontinuity, and the distance between the post and the other discontinuity is calculated. The simulated result shows good agreement with the experimental result by Marcuvitz.