We　investigate　the　thermal　instability　of　a　smooth　equilibrium　state,　in　which　the　density　function　satisfies　Schwarzschild＇s　(instability)　condition,　to　a　compressible　heat-conducting　viscous　flow　without　heat　conductivity　in　the　presence　of　a　uniform　gravitational　field　in　a　three-dimensional　bounded　domain.　We　show　that　the　equilibrium　state　is　linearly　unstable　by　a　modified　variational　method.　Then,　based　on　the　constructed　linearly　unstable　solutions　and　a　local　well-posedness　result　of　classical　solutions　to　the　original　nonlinear　problem,　we　further　construct　the　initial　data　of　linearly　unstable　solutions　to　be　the　one　of　the　original　nonlinear　problem,　and　establish　an　appropriate　energy　estimate　of　Gronwall-type.　With　the　help　of　the　established　energy　estimate,　we　finally　show　that　the　equilibrium　state　is　nonlinearly　unstable　in　the　sense　of　Hadamard　by　a　careful　bootstrap　instability　argument.