We investigated the isothermal section of the CaO-Al2O3-Sc2O3 ternary system at 1773 and 1873 K for 24 hours in Ar, and quenched in water to determine the operative phase equilibrate. The composition of the phases in equilibrium was determined by electron probe microanalysis. The isothermal section phase diagram of two temperature points (1773 and 1873 K) is obtained. The 1773 K isothermal section consists of one liquid compound (L), six binary compounds (CaO+L, Ca2Sc6Al6O20+L C(3)A+L, CaO.Sc2O3+L, CA+L, Ca2Sc6Al6O20+Sc2O3) and seven ternary compounds (Ca2Sc6Al6O20+Sc2O3+CA(6), Ca2Sc6Al6O20+Sc2O3+L, Ca2Sc6Al6O20+CA+L, Ca2Sc6Al6O20+CA(2)+CA, Ca2Sc6Al6O20+CA(2)+CA(6), CaO.Sc2O3+L+Sc2O3, C(3)A+CaO+L). At 1873 K, we found one liquid compound (L), five binary compounds (CaO+L, Ca2Sc6Al6O20+L, CaO.Sc2O3+L, CA+L, Ca2Sc6Al6O20+Sc2O3) and six ternary compounds (Ca2Sc6Al6O20+Sc2O3+CA(6), Ca2Sc6Al6O20+Sc2O3+L, Ca2Sc6Al6O20+CA+L, Ca2Sc6Al6O20+CA(2)+CA, Ca2Sc6Al6O20+CA(2)+CA(6), CaO.Sc2O3+L+Sc2O3) to exist at the isothermal section. The experimental information obtained in the present work not only is essential for the thermodynamic assessment of the CaO-Al2O3-Sc2O3 ternary system, but also is important for further investigation on separation of rare earths from metallurgical slags and rare-earth recovery.