A method for the computation of radiation patterns of microstrip antennas on substrates of finite dimensions by a combination of the mixed potential integral equation technique (MPIE) for patch antennas and the weak form of the conjugate gradient — fast Fourier transform (WCG-FFT) method for the scattering by the finite ground plane is presented. The computation is carried out in three steps. The MPIE formulation is first used for the accurate evaluation of the current distribution on the conducting surface of the patch. Using these currents as primary sources, equivalent polarisation currents in the dielectric slab are then evaluated. These two steps are based on the infinite grounded dielectric slab assumption. Finally the current distribution on the finite ground plane is determined using the WCG-FFT method. Since matrix inversion is not involved in the last step, the method is very convenient for treating small to moderate size ground planes. An alternative approach for the derivation of far field patterns of microstrip antennas on an infinite grounded dielectric slab using the volume equivalence theorem and image theory is also presented. Numerical results of radiation patterns have been verified experimentally.