Critical infrastructure systems perform functions and missions that are essential for our national economy, health, and security. These functions are vital to commerce, government, and society and are closely interrelated with people’s lives. Toprovide highly secured critical infrastructure systems, ascalable, reliable and robust threat monitoring and detection system should be developed to efficiently mitigate cyber threats. Inaddition, big data from threat monitoring systems pose serious challenges for cyber operations because an ever growing number of devices in the system and the amount of complex monitoring data collected from critical infrastructure systems require scalable methods to capture, store, manage, and process the big data. Toaddress these challenges, inthis paper, wepropose a cloud computing based network monitoring and threat detection system to make critical infrastructure systems secure. Our proposed system consists of three main components: monitoring agents, cloud infrastructure, and an operation center. Tobuild our proposed system, weuse both Hadoop MapReduce and Spark to speed up data processing by separating and processing data streams concurrently. With a real-world data set, weconducted real-world experiments to evaluate the effectiveness of our developed network monitoring and threat detection system in terms of network monitoring, threat detection, and system performance. Our empirical data indicates that the proposed system can efficiently monitor network activities, find abnormal behaviors, and detect network threats to protect critical infrastructure systems.