Combining vibration testing and numerical method is a potential inverse technique for determining elastic constants of materials because of its nondestructive characteristic, single test, and producing average properties. In order to simplify the modeling processes and to reduce complicated derivation in the numerical method, the combination of finite element analysis and optimum design is adopted in this work. A finite element package, ANSYS, is used to do the modal analysis of the composite plate. A hybrid genetic algorithm, in which a simulated annealing mutation process and adaptive mechanisms are added to the real-parameter genetic algorithm, is used to search the possible elastic constants. After obtaining the natural frequencies of the composite plates from vibration testing, this inverse technique could predict the elastic constants of the composite plate. The inverse technique is verified by comparing with other methods and by determining the elastic constants of aluminum plates, and the excellence of including the hybrid genetic algorithm is proved. The results also indicate that the present technique could obtain very accurate elastic constants of composite plates.