Title: Characterizing Malicious Edges targeting on Graph Neural Networks. Abstract: Deep neural networks on graph structured data have shown increasing success in various applications. However, due to recent studies about vulnerabilities of machine learning models, researchers are encouraged to explore the robustness of graph neural networks (GNNs). So far there are two work targeting to attack GNNs by adding/deleting edges to fool graph based classification tasks. Such attacks are challenging to be detected since the manipulation is very subtle compared with traditional graph attacks. In this paper we propose the first detection mechanism against these two proposed attacks. Given a perturbed graph, we propose a novel graph generation method together with link prediction as preprocessing to detect potential malicious edges. We also propose novel features which can be leveraged to perform outlier detection when the number of added malicious edges are large. Different detection components are proposed and tested, and we also evaluate the performance of final detection pipeline. Extensive experiments are conducted to show that the proposed detection mechanism can achieve AUC above 90% against the two attack strategies on both Cora and Citeseer datasets. We also provide in-depth analysis of different attack strategies and corresponding suitable detection methods. Our results shed light on several principles for detecting different types of attacks.