Grain boundary characteristics of Na-diffused SrTiO3 were investigated. It was observed that Na ions are segregated in grain boundaries by secondary ion mass spectroscopy analysis. Using I-V characteristics and C-V measurement, the non-linearity coefficient and the potential barrier height were found to be 15.6, 0.82 eV, respectively. The I-V characteristics, complex plane analysis, and de voltage step technique were performed to study the electrical properties of a single grain boundary. From the results of complex plane analysis, the deep electronic level was varied from 0.10 to 0.31 eV, which can be explained by the substitution of Na ions for Sr ions.