A novel planar ultrawide band (UWB) antenna based on complementary splitring resonator (CSRR) structure with bandnotched characteristics is proposed in the paper. The proposed antenna has a wide impedance matching characteristic with the gradual feeding line. By etching two splitring slots on the patch, the dual bandnotched characteristic is obtained. The fabricated antenna is tiny in size, only 35 mm×30 mm×1 mm. Simulation software HFSS 13.0 is used to analyze and optimize the structure parameters. The antenna is tested by physical measurement. The measured performance is of the same simulated results. The results show that the operating frequency band is within the range of 2.7~15.6 GHz, and the voltage standing wave ratio is lower than 2 (VSWR<2); when the operating frequency band is within the range of 3.1～3.7 GHz and 5.1～5.9 GHz, there are good bandnotched characteristics respectively in the bandwidth of WiMAX and WLAN. And there is a nearly directional radiation pattern and a stable gain. The antenna is compact and conformal, and can be applied to UWB communication system preferably.