Abstract: A cavity-backed log-periodic slot antenna with unidirectional pattern is presented in this paper for UWB radar detection system. The antenna is fed by a special coplanar waveguide (CPW) that is gradual-changed and ultra-wideband matched. In order to make the slot antenna radiate directionally, a cavity is set to the back side of it. To achieve a voltage standing wave ratio (VSWR) target which is below 2.5, a modification is proposed on the basis of rectangle cavity. Both simulated and measured results demonstrate that radiation patterns of the proposed antenna are unidirectional and quite stable within the band. Owing to the novel cavity and application of the absorbing material, the proposed antenna resonates at the lowest frequency point of 0.9 GHz as well as high gain obtained in the full frequency bands. The antenna is suitable for the applications in guidance, detection and UWB radar.