An aperture coupled circular microstrip patch antenna is analyzed. A full wave spectral domain method of moment along with a reciprocity analysis of a single aperture coupled microstrip element is applied. Both the electric surface current on the circular patch and the equivalent magnetic current on the aperture are considered. Results on effects of different feed line stub length, aperture length, different feed substrate permittivity and thickness on input impedance, resonant frequency and radiation pattern of the antenna and slot are provided