Abstract: A miniaturized circularly polarized microstrip antenna suitable for intelligent transportation system (ITS) is presented. The antenna consists of a slotted square patch and two-stage parasitic patches surrounding the square patch. Circular polarization is achieved by mode demodulation, and parasitic patches are used to increase bandwidth and improve antenna gain. The measurement results show that the proposed antenna has a good performance with -10 dB impedance band-width of 10. 9%, 3 dB axial ratio bandwidth of 2. 7%, 5. 4 dBi gain at the center frequency of 5. 8 GHz, and the cross-po- larization is not less than 21 dB. The size of antenna is 25 mm×25 mm×1. 6 mm, which is smaller than other antennas applied in ITS.