In　this　paper,　an　analysis　on　the　nonlinear　dynamics　and　chaos　of　a　simply　supported　orthotropic　functionally　graded　material　(FGM)　rectangular　plate　in　thermal　environment　and　subjected　to　parametric　and　external　excitations　is　presented.　Heat　conduction　and　temperature-dependent　material　properties　are　both　taken　into　account.　The　material　properties　are　graded　in　the　thickness　direction　according　to　a　simple　power　law　distribution　in　terms　of　the　volume　fractions　of　the　constituents.　Based　on　the　Reddy＇s　third-order　share　deformation　plate　theory,　the　governing　equations　of　motion　for　the　orthotropic　FGM　rectangular　plate　are　derived　by　using　the　Hamilton＇s　principle.　The　Galerkin　procedure　is　applied　to　the　partial　differential　governing　equations　of　motion　to　obtain　a　three-degree-of-freedom　nonlinear　system.　The　resonant　case　considered　here　is　1:2:4　internal　resonance,　principal　parametric　resonance-subharmonic　resonance　of　order　1/2.　Based　on　the　averaged　equation　obtained　by　the　method　of　multiple　scales,　the　phase　portrait,　waveform　and　Poincare　map　are　used　to　analyze　the　periodic　and　chaotic　motions　of　the　orthotropic　FGM　rectangular　plate.　It　is　found　that　the　motions　of　the　orthotropic　FGM　plate　are　chaotic　under　certain　conditions.