©　2018　the　Owner　Societies.　Organic　gain　materials　are　highly　attractive　for　lasing　due　to　their　chemical　tunability　and　large　stimulated　emission　cross　sections.　In　previous　reports,　the　radiative　decay　rate　kr　was　considered　as　an　important　factor　to　determine　outstanding　amplified　spontaneous　emission　(ASE)　performance　of　organic　molecules.　In　this　study,　we　use　quantum　mechanics　to　reveal　the　influential　factors　on　photophysical　properties　of　organic　emitters,　and　give　insight　into　the　effect　of　kr　on　ASE　performance.　Based　on　the　theoretical　analysis　of　three　molecules　with　similar　structure,　calculated　results　show　that　large　kr　derives　from　enhanced　transition　density　ρ　between　the　electronic　wave　functions　of　the　ground-state　and　the　lowest　excited　singlet　state　as　well　as　a　handful　of　low-frequency　torsional　modes　with　small　Huang-Rhys　factor　S,　further,　kr　values　are　calculated　depending　on　molecular　vibration　terms.　In　addition,　through　the　analysis　of　non-radiative　decay　rate　knr　considering　vibration　terms　(vibronic　coupling　constants),　the　comparison　of　kr　and　knr　shows　that　the　radiative　decay　process　is　promoted　in　the　three　molecules.　The　two　aspects　are　desired　for　outstanding　ASE　performance.　Our　work　shows　that　the　roles　of　transition　density　and　vibrational　modes　are　crucial　to　clarify　the　photophysical　properties,　which　govern　the　ASE　performance　in　organic　light　emitters.