Title: A frequency domain analysis of gradient-based adversarial examples. Abstract: It is well known that deep neural networks are vulnerable to adversarial examples. We attempt to understand adversarial examples from the perspective of frequency analysis. Several works have empirically shown that the gradient-based adversarial attacks perform differently in the low-frequency and high-frequency part of the input data. But there is still a lack of theoretical justification of these phenomena. In this work, we both theoretically and empirically show that the adversarial perturbations gradually increase the concentration in the low-frequency domain of the spectrum during the training process of the model parameters. And the log-spectrum difference of the adversarial examples and clean image is more concentrated in the high-frequency part than the low-frequency part. We also find out that the ratio of the high-frequency and the low-frequency part in the adversarial perturbation is much larger than that in the corresponding natural image. Inspired by these important theoretical findings, we apply low-pass filter to potential adversarial examples before feeding them to the model. The results show that this preprocessing can significantly improve the robustness of the model.