Reliability of thermal oxides grown on the n-type 4H-SiC substrates implanted by nitrogen ion with low doping levels equal to or less than 1x1018 cm-3 has been investigated. The surface morphology becomes rough by the nitrogen implantation and the post implantation annealing. The field-to-breakdown value decreases with increase in the nitrogen concentration. The average EBD values are 11.6 MV/cm, 11.3 MV/cm and 10.7 MV/cm for the samples without the implantation and with the nitrogen implantation of doping levels of 1x1017 cm-3 and 1x1018 cm-3, respectively. The time-to-breakdown values were also degraded with the increase of the nitrogen implantation doping level. The reliability degradation of thermal oxides is caused by the implantation-induced breakdown factor.