Abstract: The evolution mechanisms of MgO-Al₂O₃ in the process of calcium treatment in non-oriented silicon steel were studied by laboratory experiments, with focus on the evolution process in the early and middle of calcium treatment. A small amount of MgO-Al₂O₃ surrounded by AlN was occasionally detected besides many pure spinels before calcium treatment. CaS or CaO formed as a tran-sient inclusion in the MgO-Al₂O₃ surface 1.5 min after calcium treatment. The composition from the inner part to outer part of inclu-sions was MgO-Al₂O₃, CaO-MgO-Al₂O₃ and CaO-Al₂O₃ in order 4.5 min after calcium treatment. 10 min after calcium treatment, unreacted MgO-Al₂O₃ spinels disappeared, and simultaneously CaO-MgO-Al₂O₃ and CaO-Al₂O₃ inclusions became homogeneous in composition. The phase stability diagrams of Al₂O₃/MgO-Al₂O₃/MgO and MgO-Al₂O₃/xCaO·yAl₂O₃ were calculated using thermo-dynamic data. The result showed that it was difficult to modify MgO-Al₂O₃ spinels with very high aluminum content. In combination with the scanning electron microscopy images, energy-dispersive spectra and element mappings of typical inclusions, the evolution route of an inclusion was described as MgO or Al₂O₃→MgO-Al₂O₃→CaO-MgO-Al₂O₃→CaO-Al₂O₃, and the alumina content occa-sionally showed a sudden increase besides a common decrease in the middle part of the product.