Abstract: SiO2-CaO-Al2O3 ternary complex oxide inclusions are one of the common types of inclusions in steels. However, the chemical and physical properties of these composite inclusions, such as electronic, mechanical and thermomechanical properties, have not been sufficiently investigated. In this study, first-principles density functional theory (DFT) calculations were used to compute the properties of the commonly found SiO2-CaO-Al2O3 ternary inclusions in steel. Our electronic density of states (DOS) analysis and band structure calculations have revealed the significant role of oxygen atoms in electron reactivity, a finding that profoundly impacts our understanding of the electronic behavior of these inclusions. The mechanical properties calculated include a Young's modulus of 101.32 GPa for anorthite and 131.43 GPa for gehlenite, along with Poisson's ratios of 0.295 and 0.285, respectively. These values clearly delineate the contrasting mechanical responses exhibited by these phases. Thermodynamically, anorthite has a Debye temperature of 536.7 K, and gehlenite has 588.7 K, and the anisotropy in wave velocity provides new insights into the thermal vibration characteristics of these materials, which is essential for predicting their performance across different temperature regimes. The coefficients of thermal expansion of both anorthite and gehlenite increase rapidly with increasing temperature. However, as the temperature increases, the trend of increase of both materials starts to slow down. At the high temperature of 2000 K, the coefficients of thermal expansion for anorthite and gehlenite are approximately 12×10-6 K-1 and 8.5×10-6 K-1. These findings enhance our understanding of the physical nature of ternary inclusions in steel and provide a scientific foundation for optimizing the performance of steel materials, thereby contributing to the development of materials with superior mechanical integrity and thermal stability.