This paper presents a planar microstrip-fed monopole antenna for ultra wideband (UWB) applications. This antenna is based on the planar rectangle monopole antenna. The characteristic of the antenna can be improved by altering the figure of the radiation patch and the ground. HFSS is used to conduct simulation calculation so as to get various performance parameter of the antenna, to make sure that its frequency band range is 3.1GHz∼10.6GHz, and to meet the design requirement of the ultra-wideband antenna. The antenna size 32mm × 38mm × lmm, and it realizes the miniaturization of the antenna.