The dynamic properties of composite plates are analyzed by the boundary element method in this study. The composite plate is defined in this analysis by combining an infinite thin plate and a finite thin plate, that is casted into the infmite plate, and whose material properties are different from the infmite plate. To investigate the dynamic response of the composite thin plate, numerical analyses for some models are carried out The results show the dynamic response of the composite plates well, and also confirm the usefulness of the proposed method for the composite plate problems. Furthermore, the proposed method was applied to numerical experiment of evaluation for a defect in thin plate. Results of numerical experiment on these models show that this nondestructive evaluation method has high appropriateness and usefulness to inspect defect in thin plate.