Gravity-pouring of a TiAlNb alloy from an induction skull melting (ISM) furnace into a ceramic shell mould has been carried out in this paper. Microstructure and gas defects were examined by employing scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) analysis. It has been found that oxide films existed in the form of bubbles and porosity, and acted as defects’ heterogeneous nucleation sites. The bubbles consisted of entrained and precipitable ones, while the porosity consisted of layer and centerline ones. The diameter of casting bars, mould temperature and the partial pressure of argon had an effect on the formation of gas defects.