Acceleration factors in acceleration life test of thermal oxides grown on 4H-SiC(0001) wafers and influences of dislocations on oxide reliability have been investigated using time-dependent dielectric breakdown measurements. The thermal oxides are formed by dry oxidation at 1200°C followed by annealing in nitrogen atmosphere. Then, post oxidation annealing in wet ambient at 950°C or hydrogen atmosphere at 800°C were carried out for some of the oxides. Aluminum or poly-Si films with thickness of 300 nm were formed as gate electrodes. The temperature dependence of time-to-breakdown (tBD) indicates that activation energy (Ea) values for the Al-gate and Poly-Si-gate thermal oxides are 0.59 eV - 0.79 eV and 0.34 eV - 0.72 eV, respectively. Analyzing the electric field dependence of tBD, it was indicated that the values of electric acceleration parameters (β) are 2.7 cm/MV - 7.0 cm/MV and 5.8 cm/MV - 7.1 cm/MV for the Al-gate and poly-Si-gate thermal oxides, respectively. In addition, the charge-to-breakdown decreases with increase in the density of basal plane dislocation.