In this paper, a small size ultra-wideband (UWB) monopole antenna with band rejection characteristics is proposed. The proposed antenna can be used to reject the frequency band of wireless local area network (WLAN) or that of worldwide interoperability for microwave access (WiMAX) to overcome the interference between the UWB applications and these technologies. This antenna has a small size of 12×18 mm 2 and is matched properly to a 50-ohm line over the UWB spectrum. The proposed antenna is simulated, fabricated, and its performance is tested. A good agreement is achieved between the simulation and measurement results. Effects of the design parameters on the performance of the antenna have been investigated showing that the antenna performance has a high degree of controllability. Based on the variation of some design parameters, the proposed antenna can reject one of the two frequency bands; WLAN band or WiMAX band.