X-ray mammography is widely used for detection of breast cancer. Besides its popularity, this method did not have the potential of discriminating a tumor covered with limestone from a pure limestone mass. This might cause misdetection of some tumors covered with limestone or unnecessary surgery for a pure limestone mass. In this study, Ultra-Wide Band (UWB) signals are used for the imaging. A feed-forward artificial neural network (FF-ANN) is used to classify the mass in the breast whether it is a tumor or not by using the transmission coefficients obtained from UWB signals. A spherical tumor covered with limestone and pure limestone masses were designed and placed into the fibro-glandular layer of breast model using CST Microwave Studio Software. The radius of the masses for both cases is changed from 1 mm to 10 mm with 1 mm steps. Horn antennas were chosen to send and receive Ultra-Wide Band (UWB) signals between 2 and 18 GHz frequency range. The obtained results show that the proposed method, on the contrary of the mammogram, has the potential of discriminating the tumor covered with limestone from the pure limestone, for the mass sizes of 7, 8 and 10 mm. Consequently, the UWB microwave imaging can be used to distinguish these cases from each other.