Basal plane dislocations (BPDs) which consist of a pair of patrial dislocations (PDs) with Shockley-type stacking faults (SFs) between them are suspected to cause the forward voltage degradation (bipolar deg-radation), thus leading to the degradation of SiC-based electron devices. However, microscopic reason for the forward voltage degradation has not been fully under-stood yet. In this study, we theoretically studied the atomic and electronic structures of BPDs by using first principal calculations based on the density-functional theory (DFT) and clarified the atomistic origin of for-ward voltage degradation of SiC devices. We have found that generated stacking-faults induce gap states near conduction band bottom. Moreover, the pair of PDs also cause gap states. These gap states are the atomistic origin of the degradation of the SiC-based electron devices.