A　new　cationic　Ir(III)　complex　with　aldehyde　and　amino　groups　was　synthesized　and　characterized.　The　Ir(III)　complex　has　rich　photophysical　properties.　The　reaction　of　the　aldehyde　group　in　the　Ir(III)　complex　with　homocysteine　(Hcy)　afforded　thiazinane　derivatives　which　resulted　in　obvious　changes　in　the　luminescence　spectra.　After　addition　of　Hcy　to　the　Ir(III)　complex　containing　4,4＇-diamino-2,2＇-bipyridine,　the　luminescence　intensity　at　ca.　580-610　nm　decreased,　and　a　new　band　at　ca.490-520　nm　appeared　and　enhanced　strongly　with　a　large　blue　shift　of　ca.90　nm,　and　the　luminescent　color　changed　from　orange　red　to　green.　Based　on　this　ratiometric　probe,　it　can　sensitively　and　selectively　recognize　Hcy　by　the　ratio　of　emission　intensity　at　two　wavelengths　to　the　concentrations　of　Hcy.　While　after　addition　of　cysteine　(Cys)　or　glutathione　(GSH),　the　luminescence　band　showed　a　mild　decrease　in　intensity　with　an　unnoticeable　shift.　These　different　phenomena　make　it　capable　of　discriminating　homocysteine　from　cysteine　and　glutathione.　The　cytotoxicity　and　imaging　of　the　complex　were　also　studied　in　this　work.　The　complex　exhibited　very　low　cytotoxicity　on　HeLa　cells　and　showed　sensitivity　toward　Hcy　in　living　cells.　These　advantages　provide　it　a　good　candidate　for　the　application　in　the　analytical　and　bioanalytical　field.