The dielectric properties, including the DC breakdown strength, of 1 mol% Nb5+-doped BaTiO3 ceramics with different quantities of excess TiO2 have been investigated. The breakdown strength was found to decrease with increasing TiO2 content, but could not be readily explained by relative density and grain size effects. The decrease in the breakdown strength from a stoichiometric BaTiO3 composition to samples with excess TiO2 is believed to be due to the field enhancement effect (up to a factor of 1.40) at the BaTiO3 matrix because of the presence of a Ba6Ti17O40 second phase. The thermal expansion coefficient mismatch between the BaTiO3 matrix phase and the Ba6Ti17O40 phase may also result in a low breakdown strength. The dielectric properties of the pure Ba6Ti17O40 phase were also investigated and are reported herein.