High　velocity　arc　spraying　(HVAS)　process　was　used　to　deposit　Al-Ni-Y-Co　amorphous　and　nanocrystalline　composite　coating　on　AZ91　magnesium　alloy　substrate.　The　microstructure　of　the　coating　was　characterized　using　X-ray　diffraction　(XRD),　scanning　electron　microscopy　(SEM)　equipped　with　energy　dispersive　spectrometer　(EDS)　and　transmission　electron　microscopy　(TEM).　The　results　show　that　the　coating　with　thickness　of　500　mu　m　presents　a　dense　layered　structure　with　low　porosity　(1.8%).　The　coating　consists　of　amorphous,　nanocrystalline,　and　crystalline　phases.　The　microhardness　was　measured　by　a　microhardness　tester.　The　average　values　of　Vickers　hardness　for　the　Al-Ni-Y-Co　coating,　pure　Al　coating　and　AZ91　alloys　are　about　HV0.1　310,　HV0.1　70,　and　HV0.1　60,　respectively.　The　electrochemical　corrosion　resistance　of　the　coating　in　5　wt%　NaCl　aqueous　solution　was　also　investigated,　and　the　results　showed　that　the　Al-Ni-Y-Co　coating　exhibits　better　corrosion　resistance　than　pure　Al　coating　and　AZ91　magnesium　alloy.