Abstract: In this paper, a tri band microstrip antenna applied in intelligent transport system (ITS) is designed,which operates at GPS(1. 575GHz), WLAN(2. 45GHz) and DSRC(5. 8GHz). The wide frequency ratio is achieved by using the stacked structure, the top patch can perform DSRC, and the edges of the bottom patch with four equal slots can realize dualband. By cutting a pair of narrow slots at the edge of the square patch, the antenna can perform circularly polarized radiation. The impedance matching in three bands is achieved by adjusting the feed position, shifting the top patch, and changing the size of via hole in bottom patch. The results show good agreement between simulation and measurement, and the antenna has good performance on impedance matching, axial ratio and radiation pattern, which can meet the requirement.