Abstract: A compact ultra-wideband microstrip antenna is presented in the paper. The radiating element of this antenna is made up of semicircle plane and rectangle planes, which are fed by microstrip line. The same window of Defected Ground Structre (DGS) is etched on the back of the microstrip antenna. The performance of the antenna is simulated and optimized by CST. The simulate result shows that the relative bandwidth of the designed antenna is over 144.9% (2.15~13.47GHz), and the return loss is less than -10dB. The average gain is over 4dB in the operating range. The radiation pattern is remained almost same type in the operating range. In addition to small size and simple shapes, the antenna is also easier to be fabricated and integrated. Finally, a sample antenna is fabricated and the performance is tested. The measured result shows a good agreement with the simulated one. The experimental results show that the microstrip antenna has the advantages of small size and UWB characteristic.