A protein with long polypeptide chain always folds into more than one distinct, compact globular regions that have been designated as domains. Being one of the structural hierarchy, domain is a structural level situating in between secondary and tertiary structure. It is a basic unit to compose three-dimensional structure of protein, to execute the biological function as well as to fold a functional protein. As structural modules, domains may have advantage to compose complex proteins during the process of protein evolution by insertion, deletion and excising. Studies on the structure and interaction of domains can throw light on the study of the relationship between protein structure and function and the design of novel proteins.