We introduce a new application of the genetic algorithm for design of the coaxial fed circularly polarized rectangular microstrip antenna. The GA shows excellent results for the design of the microstrip antenna which have a very complex objective function including the input impedance, the effective loss tangent and the axial ratio. The objective function is derived from the cavity model, and the size and the feeding point are optimized by the genetic algorithm. The experimental results show good agreement with the simulated results.