Self-diffusion coefficients of calcium in molten slags (CaO-SiO_2, CaO-Al_2O_3, CaO-SiO_2-Al_2O_3 and CaO-SiO_2-Al_2O_3-MgO systems) were measured by the method of semi-infinite medium in the temperature range from 1350°to 1600℃. The diffusion coefficients were of the order of 10^6～10^7cm^2 sec^ and the activation energies were 40～70kcal/g・mol for all slags investigated. The diffusion coefficient has good correspondence to both the specific electric conductivity and the viscosity coefficient. Some behaviors of Al_2O_3 and MgO in basic slags were also discussed
