In this paper, the effect of shorting wall length on the circular polarization of shorted microstrip patch antenna is examined. The theoretical analysis is based on the High Frequency Software Simulator (HFSS®). A designed antenna for the GPS application at the civilian GPS frequency (1.575 GHz) has been fabricated and measured. The measured data for the fabricated antenna is taken by 8719 ES vector network analyzer. Results of the designed antenna show good circularly polarized with a reduction in its size by 24.6 % from the conventional rectangular microstrip patch antenna. Simulated and measured results for the proposed antenna are presented.