An ultra-wideband (UWB) microwave robust Capon beamforming imaging system is presented based on a self-designed compact UWB antenna for early breast cancer detection. Simulation results show that the proposed detection system is sensitive to tumor response. Simulated tumor detection experiments are carried out in both simple planar breast model and magnetic resonance imagining (MRI)-derived model using the antenna array. Robust Capon beamforming algorithm is employed to reconstruct the breast image. Successful detection of 3-mm-diameter tumor is achieved in the planar model and the same detection result of 4-mm-diameter tumor is achieved in the MRI-derived model. The right information of the tumor can be obtained from the imaging results, which demonstrates the feasibility of the proposed system in early breast cancer detection.