In this study the electrical conductivity of excess BaO in $BaTiO_3$ was measured to investigate the relationship between defects and solubility in the temperature range of $900^{circ}C$ to $1300^{circ}C$ under various oxygen partial pressure. First of all, quenched $BaTiO_3$ powders of various Ba/Ti ratios were analysed by X-ray diffraction to confirm whether second phase is formed or not. As the results, we observed the solubility of BaO in the temperature range of $1200^{circ}C$ to $1400^{circ}C$, and it was also found that the conductivity minima move to lower $PO_2$ with increasing excess BaO within solubility limit.