Formation energies of various defects in Al2O3 and SiO2 are calculated by using the plane-wave pseudopotential method. Also, the formation energies of Schottky defects and Frenkel defects are evaluated on the basis of these calculations. It is shown that formation energies of these defects are higher in SiO2 than in Al2O3. In other words, less defects are formed in SiO2 than in Al2O3. It is also found that the principal defect is the cation Frenkel defect in Al2O3 but the anion Frenkel defect in SiO2. These results agree with the experimental results that Al ions diffuse preferably in Al2O3 but oxygen ions diffuse in SiO2 at high temperatures.