A miniaturized planar UWB (Ultra Wide Band) antenna implemented on a FR-4 substrate is proposed in this paper. The antenna uses a tapered microstrip line as a feed line, and has 1-stage trapezoid shape ground. Its total size is 25 × 30 mm2. The antenna proposed in this paper is much smaller than the antenna using a uniform feed line and a conventional square ground. It is also smaller than the UWB antenna we proposed last year. The simulated and measured results on impedance matching, radiation pattern and time domain characteristic over UWB band are given.