Abstract: An ultra wideband slot antenna with dual band notched characteristics improved by complementary splitring resonator (CSRR) is proposed due to that UWB system is easy to be interfered with the narrow band signals. In the published literatures, a single CSRR can achieve only one notch band, two or more CSRRs are required to increase other notch bands, which increases the complexity of the design. The corresponding relations between the inner and outer rings of CSRR and the two notch bands were analyzed by means of the surface current distributions on the radiation patch. The proposed antenna was fabricated and measured, it is in good agreement between simulations and measurements, which verify the correction of the design