In this paper, a small size ultrawideband (UWB) planar antenna is proposed. This antenna is a U-shaped monopole antenna which is fed using a microstrip line feeder. The size of the antenna has been miniaturized by optimizing the dimensions of the U-shaped radiator and the ground plane. The final size of the antenna after optimization of its dimensions is 12mm×18mm×1.6mm. The proposed antenna is simulated, fabricated, and its performance is tested while satisfactory performance is achieved. The comparison between the simulation and measurement results shows good agreement.