The carbide capacity of the CaO–SiO₂–MnO slag, which is the main system produced during Mn alloys processes, through the wide composition region has been measured at 1773 K to understand the effective slag composition on the solubility of carbon in molten slags during SiMn production processes. The carbide capacity is strongly affected by slag composition and this tendency can be reasonably estimated by employing the activity of lime and the activity coefficient of CaC₂ as a thermodynamic factors affecting carbide capacity. Considering the high concentration of MnO during SiMn smelting process, the lime to silica ratio of 0.8 (±0.1) is recommended in view of high ability of carbon dissolution. The carbide capacity of the CaO–SiO₂–MnO slag can be expressed as a linear function of the activity of lime and the optical basicity. The carbide capacity of the CaO–SiO₂–MnO slag increases more significantly than the sulfide capacity does as the basicity of the slag increases.