Fluorination of Al2O3 and Y2O3 surfaces was investigated by irradiating high-density, helicon-wave CF4/O-2 and SF6/O-2 plasmas. The Al2O3 surface bombarded by high-flux positive ions of the CF4/O-2 plasma was fluorinated significantly. On contrast, Y2O3 was less fluorinated than Al2O3 when they were irradiated by the same CF4/O-2 plasma. The analysis of the Al2O3 surface irradiated by the CF4/O-2 plasma suggests that the fluorination is triggered by reactions between fluorocarbon deposit and Al-O bonding with the assistance of ion bombardment. On the other hand, irradiation of the SF6/O-2 plasma induced less significant fluorination on the Al2O3 surface. This suggests a lower reaction probability between sulfur fluoride deposit and Al-O bonding. The difference in the fluorination of the Al2O3 and Y2O3 surfaces induced by the irradiations of the CF4/O-2 and SF6/O-2 plasmas is understood by comparing the bonding energies of C-O, S-O, Al-O, and Y-O.