An ultra-wide band planar monopole antenna is presented in this paper. This antenna uses a pentagonal patch as radiation unit and is fed by a microstrip line. The relevant simulation is performed by using Ansoft HFSS. The return loss curve and the radiation patterns of this new antenna are given. And the effect of the size of the various parts of the antenna on the reflection loss is analyzed. Experimental results show that the proposed antenna provides an ultra-wide bandwidth (1.70~12.90 GHz) for return loss less than -10dB (VSWR<2). It is suitable for current ultra-wide band communication systems.