Through　simultaneously　enhancing　the　power　factor　by　engineering　the　extra　light　band　and　enhancing　phonon　scatterings　by　introducing　a　high　density　of　stacking　faults,　a　record　figure-of-merit　over　2.0　is　achieved　in　p-type　AgSbTe2-xSex　alloys.　Density　functional　theory　calculations　confirm　the　presence　of　the　light　valence　band　with　large　degeneracy　in　AgSbTe2,　and　that　alloying　with　Se　decreases　the　energy　offset　between　the　light　valence　band　and　the　valence　band　maximum.　Therefore,　a　significantly　enhanced　power　factor　is　realized　in　p-type　AgSbTe2-xSex　alloys.　In　addition,　transmission　electron　microscopy　studies　indicate　the　appearance　of　stacking　faults　and　grain　boundaries,　which　together　with　grain　boundaries　and　point　defects　significantly　strengthen　phonon　scatterings,　leading　to　an　ultralow　thermal　conductivity.　The　synergetic　strategy　of　simultaneously　enhancing　power　factor　and　strengthening　phonon　scattering　developed　in　this　study　opens　up　a　robust　pathway　to　tailor　thermoelectric　performance.