Silver-based　I-III-VI-type　semiconductor　nanocrystals　have　received　extensive　attention　due　to　their　narrow-band　luminescence　properties.　Herein,　we　demonstrated　a　seed-mediated　growth　of　quaternary　Ag-In-Ga-S　(AIGS)　nanocrystals　(NCs)　with　narrow-band　luminescence.　By　conducting　partial　cation　exchange　with　In3+　and　Ga3+　based　on　Ag2S　NCs　and　controlling　the　Ag/In　feeding　ratios　(0.25　to　2)　of　Ag-In-S　seeds　as　well　as　the　inventory　of　1-dodecanethiol,　we　achieved　optimized　luminescence　performance　in　the　synthesized　AIGS　NCs,　characterized　by　a　narrow　full　width　at　half　maximum　of　less　than　40　nm.　Meanwhile,　narrow-band　luminescent　AIGS　NCs　exhibit　a　tetragonal　AgGaS2　crystal　structure　and　a　gradient　alloy　structure,　rather　than　a　core-shell　structure.　Most　importantly,　the　kinetics　decay　curves　of　time-resolved　photoluminescence　and　the　ground　state　bleaching　in　transient　absorption　generally　agree　with　each　other　regarding　the　lifetime　of　the　second　decay　component,　which　indicates　that　the　narrow-band　luminescence　is　due　to　the　slow　radiative　recombination　between　trapped　electrons　and　trapped　holes　located　at　the　edge　of　the　conduction　band　and　the　deep　silver-related　trap　states　(e.g.,　silver　vacancy),　respectively.　This　study　provides　new　insights　into　the　correlation　between　the　narrow-band　luminescence　properties　and　the　structural　characteristics　of　AIGS　NCs.　Narrow-band　luminescence　was　observed　from　Ag-In-Ga-S　alloyed　nanocrystals　were　synthesized　through　a　seed-mediated　growth　and　cation　exchange　strategy,　which　was　originated　from　the　recombination　of　the　conduction　band　edge　to　silver　vacancy　states.