Abstract. In pattern matching based Protein-Protein Interaction Extraction systems, patterns generated manually or automatically exist erroneous and redundancy, which greatly affect the system’s performance. In this paper, a MDLbased pattern optimizing algorithm is proposed to filter out the bad patterns and redundancy. Experiments show that our algorithm is effective in improving the system’s performance while greatly cutting down the number of patterns. It also has excellent generalizability which is important in implementing practical systems.