Honeycomb sandwich structures have been widely used in many engineering fields because of their excellent mechanical properties. The formulas for the cantilever honeycomb sandwich plate are derived, and the nonlinear vibrations of the plate are given in this paper. In order to obtain the equivalent elastic parameters of the hexagonal core layer in the honeycomb sandwich plate, the equivalent elastic parameters that more closer to the finite element solutions for the cores are selected. Based on the Reddy′s third?order shear deformation theory, the nonlinear partial differential equations of motion are derived for the composite laminated cantilever plate subjected to in?plane and transverse excitations by using the Hamilton＇s principle. The Galerkin method is then used to transform the nonlinear partial differential equations of motion to a two?degree?of?freedom nonlinear system ordinary differential equation of motion. The numerical method is also utilized to examine the nonlinear dynamic responses of the cantilever honeycomb sandwich plate. The results show that in?plane and transverse excitations have an important influence on nonlinear dynamic characteristics, and periodic, multi?periodic, quasi?periodic motions and chaotic motions all occur for the system with the change of forcing loads.