In recent years, Distributed Denial-of-Service (DDoS) and Denial-of-Service (DoS) are the most dreadful network threats. Single-node IDS often suffers from losing its detection effectiveness and capability when processing enormous network traffic. To solve the drawbacks, we propose a Grid-based IDS, called Grid Intrusion Detection System (GIDS), which uses Grid computing resources to detect intrusion packets. For balancing detection load, Score Subtraction Approach (SSA) and Score Addition Approach (SAA) are deployed. Furthermore, to effectively detect intrusions, a two-phase packet detection process is proposed. The first phase detects logical and momentary attacks. Chronic attacks are detected in the second phase. Experiments are also performed and the results show that GIDS is truly an outstanding system in detecting attacks.