The effects of TiO2 addition on the electrical insulation of AlN ceramics with 1 wt% Y₂O₃ as a sintering aid have been investigated. Some of TiO₂ has reacted with AlN powders and transformed to fine TiN particles during sintering, which was uniformly dispersed along grain boundaries of AlN. At a high electrical field (500 V/mm), the resistivity of AlN ceramics with TiO₂ addition of 0.2 wt% increased about 1000 times from 3x10¹⁰ Ω cm to 3.1×10¹³ Ωcm. Based on the impedance spectroscopy measurement, it was found that TiO₂ addition increased dramatically electrical resistivity of AlN grains much more than that of grain boundaries. Thus, TiO2 was believed to dissolve inside AlN grains to suppress ionic conduction of Al vacancies. This suppressed ionic conduction by Ti incorporation into AlN grains seems to contribute to more electrically insulating AlN ceramics.