In this paper the numerical computation of resonant frequency of gap coupled circular microstrip antennas using the concept of cavity model has been presented. The resonant frequency of the gap-coupled circular patches is computed and the mode number is also evaluated. The computed results are compared with published results. The previously published results are based on experimentation and do not present a clear mathematical model for the same structure. An extensive comparison with Method-of-moments based software (IE3D) is also done. The accuracy of prediction is reasonably good.