The modal properties of continuous active structures are studied in this paper. The reciprocity theorem is derived and presents the relationship between the two co-adjoint active structures. The modal orthogonality conditions of the active plate and active beam with distributed sensor and actuator are developed. The continuous active structures and their adjoint active structures have the same eigenvalves, and their modes are weighted orthogonal. The present study concludes that the modes of active structures are related to the control law, measurement and actuation configurations. As a numerical example a rectangular plate was calculated to give the notes about modes of the active structures.