Title: Empirical Study of the Decision Region and Robustness in Deep Neural Networks. Abstract: In general, the Deep Neural Networks (DNNs) is evaluated by the generalization performance measured on the unseen data excluded from the training phase. Along with the development of DNNs, the generalization performance converges to the state-of-the-art and it becomes difficult to evaluate DNNs solely based on the generalization performance. The robustness against the adversarial attack has been used as an additional metric to evaluate DNNs by measuring the vulnerability of them. However, few researches have been performed to analyze the adversarial robustness in terms of the geometry in DNNs. In this work, we perform empirical study to analyze the internal properties of DNNs which affect model robustness under adversarial attacks. Especially, we propose the novel concept Populated Region Set (PRS) where train samples populated more frequently to represent the internal properties of DNNs in the practical setting. From the systematic experiments with the proposed concept, we provide empirical evidences to validate that the low PRS ratio has strong relationship with the adversarial robustness of DNNs.