Abstract. Flow-based intrusion detection systems analyze IP flow records to detect attacks against computer networks. IP flow records contain aggregated packet header information; therefore, the amount of data processed by the intrusion detection system is reduced. In addition, since no payload is analyzed, the end-to-end encryption does not affect the deployment of intermediate intrusion detection system. In this paper, we evaluate one-class classification techniques for detection of malicious flows at an initial stage of a multi-stage flow-based intrusion detection system. The initial stage uses minimal flow attributes and only decide if the IP flow is normal or malicious. Since there is only one class of interest (malicious) at the initial stage, we use one-class classification for detection of malicious flows. In this paper, we review available one-class classification techniques and evaluate them on a flow-based dataset to determine their performance for detection of malicious flows. Our results show that one-class classification techniques using boundary methods give best results in detection of malicious IP flows.