The distinguished characteristic of casting process of natural yellow clay-bonded green sand is a strippable sintering layer formed at the interface of casting and mould. Hot shake-out can be realized because this sintering layer can firmly attach to the casting surfaces to protect castings at elevated temperature. Better surface quality of castings is achieved after the sintering layer easily shed from the casting surface at room temperature. In this study, a series of tests were carried out to reveal the characteristic of this sintering layer. First, thermal gravimetric/differential thermal analysis (TG/DTA) method was used for detailed analysis the thermal property of natural yellow clay-bonded green sand. And, X-ray diffraction (XRD) analysis was employed to define the phase composition at variable temperatures. Then, sintering experiments were carried out on mixtures of natural yellow clay-bonded sand with iron powder or iron oxides under different conditions. Finally, the mechanical performances of sinter layer of natural yellow clay-bonded sand at elevated and room temperatures were investigated. The result revealed that natural yellow clay-bonded green sand is a kind of low grade molding sand, and iron oxides participate in the forming of sintering layer. The sintering layer of natural yellow clay-bonded sand were proved both excellent plastic at elevated temperature and brittle at room temperature, which coincides with its behavior in the casting process.