This paper presents the design of a Vivaldi antenna for microwave imaging applications, specifically for breast cancer detection. A number of parameters are studied and optimized for breast imaging application over ultra-wide band frequency (3-9 GHz). The overall size of the antenna is 0.57 λ x 0.406 λ at the lower frequency of 3 GHz. Advanced image reconstruction algorithms based on the reflected signal of the Vivaldi antennas are then utilized for the imaging task. The algorithms provide sharper and superior identification capability, accurate and reliable positioning, strong robustness, and efficient computational speed.