In the U.S., breast cancer is diagnosed in about 12% of women during their lifetime and it is the second leading reason for women’s death. Since early diagnosis could improve treatment outcomes and longer survival times for breast cancer patients, it is significant to develop breast cancer detection techniques. The Convolutional Neural Network (CNN) can extract features from images automatically and then perform classification. To train the CNN from scratch, however, requires a large number of labeled images, which is infeasible for some kinds of medical image data such as mammographic tumor images. In this paper, we proposed two solutions to the lack of training images. 1)To generate synthetic mammographic images for training by the Generative Adversarial Network (GAN). Adding GAN generated images made to train CNN from scratch successful and adding more GAN images improved CNN’s validation accuracy to at most (best) 98.85%. 2)To apply transfer learning in CNN. We used the pre-trained VGG-16 model to extract features from input mammograms and used these features to train a Neural Network (NN)-classifier. The stable average validation accuracy converged at about 91.48% for classifying abnormal vs. normal cases in the DDSM database. Then, we combined the two deep-learning based technologies together. That is to apply GAN for image augmentation and transfer learning in CNN for breast cancer detection. To the training set including real and GAN augmented images, although transfer learning model did not perform better than the CNN, the speed of training transfer learning model was about 10 times faster than CNN training. Adding GAN images can help training avoid over-fitting and image augmentation by GAN is necessary to train CNN classifiers from scratch. On the other hand, transfer learning is necessary to be applied for training on pure real images. To apply GAN to augment training images for training CNN classifier obtained the best classification performance.