BackgroundThe　plate　structure　is　one　of　the　typical　components　of　the　mechanical　system　and　its　vibration　research　has　attracted　full　attention.　Graphene　platelet　is　a　new　high-performance　nano-reinforced　filler　and　its　emergence　provides　an　opportunity　to　improve　the　vibration　properties　of　conventional　composite　plate　structures.PurposeThe　free　vibration　characteristics　of　the　three-phase　composite　thin　plate　reinforced　synergistically　by　graphene　platelets　(GPLs)　and　carbon　fibers　are　studied　in　this　paper,　and　we　also　analyze　the　influences　of　the　content　changes　of　different　reinforcements　on　the　natural　frequencies　and　mode　shapes　of　the　three-phase　composite　thin　plate.MethodsThe　effective　material　properties　of　the　three-phase　composite　material　are　calculated　according　to　a　combination　of　the　Halpin-Tsai　model　and　the　micromechanics　approach.　The　separation　method　of　variables　is　used　to　obtain　the　natural　frequencies　and　mode　shapes　of　the　three-phase　composite　thin　plate　under　different　boundary　conditions.　The　accuracy　of　results　from　the　separation　method　of　variables　is　verified　by　comparison　with　the　references.ConclusionsThe　synergistic　effect　of　GPLs　and　carbon　fibers　can　increase　the　natural　frequencies　of　a　composite　thin　plate　under　different　boundary　conditions.　The　mode　shape　transformation　phenomena　mostly　appear　in　the　higher　order　vibrations　of　the　three-phase　composite　thin　plate　when　the　GPLs　content　increases.　The　appearance　of　mode　shape　transformation　is　accompanied　by　a　change　in　the　rate　of　frequency　growth.　These　results　have　important　theoretical　significance　for　a　widespread　engineering　application　of　the　three-phase　composite　structure.