The circular aperture antenna which is an open-ended circular waveguide stimulated by TE11 mode is analyzed. The method of moments is employed in conjunction with the equivalent-source technique and the modal expansion theory to analyze the open-ended circular waveguide. No restriction is imposed on the number of modes in the analysis. The problem of radiation from a circular aperture antenna embedded in an infinite ground plane is solved at first. Then expressions which represent the elevation-plane patterns of the antenna embedded in a finite ground plane are derived using GTD(Geometrical Theory of Diffraction). The expressions for the field pattern are obtained by superposing the infinite ground plane solutions, the first-order diffracted field solutions and the axial caustic solutions in their respective regions of validity.