Identification of protein complexes in large interaction networks is crucial to understanding principles of cellular organisation and predict protein functions. In this paper, a new algorithm of Identifying Protein Complexes based on Maximal Clique Extension (IPC-MCE) is proposed. The maximal clique is considered as the core of the protein complex. Proteins in a complex are classed into core vertices and peripheral vertices. The relation between the core vertices and peripheral vertices is measured by the Interaction Probability. The algorithm IPC-MCE is applied to the protein interaction network of Saccharomyces cerevisiae. Many well-known protein complexes are detected.