A decomposition in sub-arrays is used to reduce the computation time in the modeling of a finite array of microstrip antennas. The surface currents are considered identical on the patches which have the same neighbourhood configuration. This simplifies the resolution of the integral equation by reducing the number of unkown coefficients. Examples are presented in the cases of linear arrays ( made up of parasitic or fed elements). The theory agree well with the experiment and this method is less time-consuming than the "element by element" approach.