Single-Event Burnout (SEB) is a radiation destruction phenomenon of power devices such as Si devices known as a catastrophic failure caused by heavy-ions. SiC power devices also exhibit SEB failure under radiation environments, however, its mechanism remains controversial. In this study, to investigate the mechanism of SEB in trench gate SiC-MOSFETs, we verified the operation of the parasitic transistor, and the effect of the epi/sub-interfacial electric field relaxation. In conclusion, our results demonstrated that electric field relaxation using a buffer layer is effective against SEB in trench gate SiC-MOSFETs.