The correlation between thermal oxide reliability and dislocations in n-type 4H-SiC (0001) epitaxial wafers has been investigated. The thermal oxides were grown by dry oxidation at 1200 °C followed by nitrogen postoxidation annealing. Charge-to-breakdown values of thermal oxides decrease with an increase in the number of the dislocations in a gate-oxide-forming area. Two types of dielectric breakdown modes, edge breakdown and dislocation-related breakdown, were confirmed by Nomarski microscopy. In addition, it is revealed that basal plane dislocation is the most common cause of the dislocation-related breakdown mode.