A method for calculating the resonant frequency of electrically thin and thick rectangular microstrip antennas, based on the backpropagation multilayered-perceptron network. is presented. The method can be used for a wide range of substrate thicknesses and permittivities. and is useful for the computer-aided design (CAD) of microstrip antennas. The calculated resonant-frequency results are in very good agreement with the experimental results reported elsewhere.