Abstarct: Being able to monitoring structural health and identifying damage at least time is important problem for the engineers especially in mechanics and aerospace. Here, one of the attractive methods for structural health monitoring is strain energy damage index method baxsed on the frequent vibration properties of structures. This method is baxsed on the fact that modal parameters of structure (such as frequency, mode shape and modal damping) are function of its physical properties (such as mass, stiffness and damping). The properties Changed are caused that the strain energy of structures changes. Hence, it can be used strain energy of damaged and undamaged structures for identifying fault position. Aim of present research is damage detection of grid stiffened composite cylindrical shells using strain energy method. First, a brief introduction of composite structures with grid stiffeners, their advantages and applications than other structures is discussed. Then, some of the common defects in these structures are discussed. In the following, an overview of the modal analysis of structures, fault detection using modal analysis and history of works done, are explained. So, the needed equations for the modal strain energy of hexagonal grid stiffened composite cylindrical shells and the strain energy damage index are extracted and used to identify damage in numerical and experimental methods. The results of damage index on the finite element and experimental models demonstrate that, despite the noise generated by the test, it can identify cracks successfully.