A novel design method of circularly polarized planar slot antenna for 5.8 GHz Dedicated Short Range Communications (DSRC) applications is proposed. For realized circularly polarized wave, we use planar slot antenna. By optimizing the lengths and widths of antenna slot, resonances can be obtained at 5.8 GHz frequency. The antenna generates a circularly polarized wave providing a slot in the two orthogonal directions. Improving the gain by providing a slot in the same direction, arrayed structure is adopted. The final antenna size is 17 mm × 15 mm × 1.6 mm. Our antenna was fabricated on the FR4 substrate and impedance bandwidth is 310 MHz from 5.62 to 5.93 GHz. Moreover circularly polarized wave can be obtained.