Title: Decision boundary variability and generalization in neural networks. Abstract: Existing works suggest that the generalizability is guaranteed when the margin between data and decision boundaries is sufficiently large. However, the existence of adversarial examples in neural networks shows that excellent generalization and small margin can exist simultaneously, which casts shadows to the current understanding. This paper discovers that the neural network with lower decision boundary (DB) variability has better generalizability. Two new notions, algorithm DB variability and $(\epsilon, \eta)$-data DB variability, are proposed to measure the decision boundary variability from the algorithm and data perspectives. Extensive experiments show significant negative correlations between the decision boundary variability and the generalizability. From the theoretical view, we prove two lower bounds and two upper bounds of the generalization error based on the decision boundary variability, which is consistent with our empirical results. Moreover, the bounds do not explicitly depend on the network size, which is usually prohibitively large in deep learning.