Abstract-A novel UWB antenna with adjustable rejection bandwidth is proposed and fabricated. The proposed antenna consists of a monopole and a novel dual-T square resonator. The results demonstrate that a wide rejection bandwidth from 10.4 to 11.5 GHz can be achieved, and the rejection bandwidth can be adjusted by transforming the dimensions of dual-T square resonator. Moreover, reflection coefficient curve with two poles at one rejected bandwidth is also obtained which is induced by the proposed novel square resonator. In addition, to explain the mechanism of adjustable rejection bandwidth, the analysis of parametric study and electric field distributions of the design are given. The total volume of the antenna is 27 mm × 19 mm × 1 mm. Compared to other recent works, a simpler structure, wider rejection bandwidth and more compact size are the key features of the proposed antenna. Owing to its adjustable bandwidth and simple structure, the proposed antenna can be used in UWB communications applications to suppress the radio-frequency interference.