Gravity-pouring　of　a　TiAlNb　alloy　from　an　induction　skull　melting　(ISM)　furnace　into　a　ceramic　shell　mould　was　carried　out.　The　microstructure　and　gas　defects　were　examined　by　scanning　electron　microscopy　(SEM),　energy-dispersive　X-ray　(EDX)　and　X-ray　photoelectron　spectroscopy　(XPS)　analysis.　Results　show　that　the　oxide　films　exist　in　the　bubbles　and　porosity,　and　act　as　defects＇　heterogeneous　nucleation　sites.　The　bubbles　consist　of　entrained　and　precipitable　ones,　while　the　porosity　consist　of　layer　and　centerline　ones.　The　diameter　of　casting　bars,　mould　temperature　and　the　partial　pressure　of　argon　influence　the　formation　of　gas　defects.