A new knowledge-based potential method, the tri-residue contact potential (TRICP), is proposed. This approach resembles the idea of environment-dependent residue contact energy reported by Zhang et al. We statistically study the 3-dimensional structure of proteins and calculate the knowledge-based contact potential of tri-residue clusters. These contact potentials are carefully compared to pairwise contact potentials (PCP). TRICP helps us to systematically examine the impact on the interactions of residue pairs by the third residue. Moreover, TRICP provides us with many clues to identify high frequency characteristic structural units in protein structures.