The properties of MnO-Al2O3-SiO2-based plasticized inclusion are likely to change during soaking process due to its low melting point. In this study, the evolution of the MnO-Al2O3-SiO2-based inclusion of 18wt％Cr-8wt％Ni stainless steel under isothermal soaking process at 1250°C for different times was investigated by laboratory-scale experiments and thermodynamic analysis. The results showed that the inclusion population density increased at the first stage and then decreased while their average size first decreased and then increased. In addition, almost no Cr2O3-concentrated regions existed within the inclusion before soaking, but more and more Cr2O3 precipitates were formed during soaking. Fur-thermore, the plasticity of the inclusion deteriorated due to a decrease in the amount of liquid phase and an increase in the high-melting-point-phase MnO-Cr2O3 spinel after the soaking process. In-situ observations by high-temperature confocal laser scanning microscopy (CLSM) con-firmed that liquid phases were produced in the inclusions and the inclusions grew rather quickly during the soaking process. Both the experimental results and thermodynamic analysis conclude that there are three routes for inclusion evolution during the soaking process. In particular, Ostwald ripening plays an important role in the inclusion evolution, i.e., MnO-Al2O3-SiO2-based inclusions grow by absorbing the newly precipitated smaller-size MnO-Cr2O3 inclusions.