An antenna with a wide bandwidth is required for ultra-wideband (UWB) system of the future. Several types of wideband antenna that cover the whole frequency range have been proposed. Since the UWB system would cover from 3.1 to 10.6 GHz, it is necessary to suppress the interference from other systems using some of this frequency band. This paper presents two types of novel planar monopole antenna: one consists of two connected rectangular plates and another one is an orthogonal type. The return loss characteristics, radiation pattern, and current distribution of these antennas were simulated by using the FDTD method. The proposed antennas had dual frequency and broad bandwidth characteristics at both resonant frequencies. The return loss level at the eliminated frequency between the resonant frequencies was almost 0 dB. The radiation patterns for the whole frequency range were almost omni-directional in the horizontal plane. The current distributions at each frequency were similar to that of a planar rectangular monopole. The radiation patterns thus were omni-directional in the horizontal plane at each resonant frequency. Therefore, the results showed that wide bandwidth characteristics could be achieved with such antennas.