Current-voltage (I-V) characteristics across Sigma 1 boundaries of Nb-doped SrTiO3, bicrystals were investigated with a special interest in the effect of an impurity ion, cobalt. The Sigma 1 boundary with cobalt ions was fabricated by hot-joining technique after evaporating metallic cobalt onto the (001) plane used for a contact plane. High-resolution transmission electron microscopy (HREM) study associated with X-ray energy dispersive spectroscopy (EDS) revealed that cobalt solved and distributed around the Sigma 1 boundary which was perfectly joined without any second films. It was Found that the Sigma 1 boundary with cobalt ions exhibited a nonlinear I-V relationship while the Sigma 1 boundary without cobalt ions showed a linear I-V relationship. This result indicates that the presence of cobalt itself at the boundary can form a potential barrier irrespective of grain boundary coherency.