This work presents a nondestructive detection of damage in thin plate and cylindrical shell structures by using modal analysis and energy index method (EIM). An aluminum alloy 6061 thin plate and cylindrical shell with a surface crack is investigated in this study. EMA is conducted on the structures to obtain the mode shapes before and after damage. The modal displacements of each mode shape are then used to compute the modal strain energy. For all measured mode shapes, a damage index is defined by using the ratio of modal strain energies of the structures before and after damage. This damage index is adopted to detect the damage location in thin plate and cylindrical shell structures. Both local and global structures are evaluated in this study. A scanning damage index (SDI) is also developed by moving damage indices obtained from the local area throughout the structure as if a scanning sensor is used to inspect the structure. The damage indices in overlap areas are added up and the summation may intensify the signals of damage in the thin plate and cylindrical shell. Limited by the numbers of measured point, a differential quadrature method (DQM) is employed to calculate the partial differential terms in strain energy formula. A finite element analysis (FEA) is also performed to access this approach and demonstrate a feasible process for the experimental work. Analytical and Experimental results show that scanning damage index well identify surface crack location by using only few measured mode shapes of the aluminum thin plate and cylindrical shell.