Abstract: Composite honeycomb plate is a class of important structure in the aerospace field. But it is rarely reported about the research on the modal characteristics of honeycomb structure in thermal environment. In this paper, modal tests on a plate composed by CF/epoxy laminates and Nomex honeycomb core were performed in six different temperature environments. During heating, the face sheet is debonded from the honeycomb core. Before delamination, natural frequencies of the plate decrease with the rising temperature. Among the first 7 modes, the decrease of the frequencies of bending modes is larger than that of the torsion modes. After debonding, the modal parameters of the plate at room temperature are quite different from the original structure. The modal damping ratio is significantly increased, and the order of two mode shapes is reversed. Lastly, the causes of the debonding of face sheet are analyzed and the conclusion is helpful to the structural design of composite honeycomb structures used in thermal environment.