The effects of dislocations in n-type 4H-SiC(0001) epitaxial wafers on the reliability of thermal oxides have been investigated. Charge-to-breakdown (Q(BD)) values of thermal oxides decrease with increase in the dislocations under a gate-oxide area. Nomarski microscope observations show that dielectric breakdown of thermal oxides occurs at the position of dislocation in epitaxial layer. It is reavealed that basal plane dislocation is the most common cause of the dielectric breakdown.