The　flutter　mechanism　for　cantilevered　beam　in　supersonic　flow　is　studied　based　on　Timoshenko　beam　model　and　piston　theory.　The　partial　differential　equations　governing　transverse　deflection　and　rotation　angle　of　the　beam　are　derived　by　analyzing　the　finite　section　of　the　beam.　Based　on　the　mode　functions　of　the　unperturbed　system,　complex　natural　frequencies　are　investigated　to　predict　the　instability　of　the　beam　by　Galerkin　truncation.　The　mechanism　of　both　coupled-mode　and　single-mode　flutter　caused　by　the　supersonic　flow　are　spotted　for　different　flow　velocity　with　consideration　of　the　shear　deformation　and　rotary　inertia　in　the　beam　structure　by　the　analytical　method.　The　instability　region　is　discussed　by　Routh-Hurwitz　criterion.　The　critical　Mach　number　for　the　panel　motion　becoming　unstable　is　calculated　for　given　values　of　the　dimensional　parameters.　(c)　2019　American　Society　of　Civil　Engineers.