On the basis of the research on a large number of instances concerning mechanical symmetry, the existing concept system of mechanical symmetry was improved and completed, and the concept and hierarchical system of mechanical symmetry classified by structure symmetry, principle symmetry and function symmetry was presented to support establishing a comprehensive mechanical symmetry concept system, systematically researching mechanical symmetry, refining design principles of mechanical symmetry, and directing applications of symmetry in mechanical design. The definitions of all kinds of symmetries were provided, and their existence and working principles in mechanical systems were illustrated by accurate instances. The application strategies of structure symmetry, principle symmetry and function symmetry on improvement and innovation of products were also illustrated. Analysis of instances showed that the concept system could completely contain all kinds of existence of symmetry in mechanical system, and it offered theoretic foundation for further research on mechanical symmetry design principles and the development of mechanical symmetry design methods.