A new approach based on artificial neural networks  for calculating the radiation efficiency of a rectangular microstrip antenna is presented. The backpropagation algorithm is used to train the networks. The method is valid for substrates with relative permittivities between 1 and 12.8 and for the complete range of thicknesses normally used. The results obtained by using this new method are in conformity with those reported elsewhere. The method can also be used in the calculation of the radiation efficiency of dipoles.