The degradation in insulation resistance under highly accelerated test conditions was investigated in terms of micro-structural and micro-chemical changes in dielectric layer of multilayer ceramic capacitor. The ceramic capacitors were prepared by using BaTiO3 powder with different size of 0.52 mu m, 0.55 mu m, and 0.58 mu m. As the particle size of BaTiO3 powder was increased, the capacitance and the dissipation factor were decreased. According to the result of highly accelerated test conducted at 150 degrees C, 75 V, and 20 h, failure in insulation resistance was increased with the particle size and the calculated FITs (failure in term) were 1.10 at 0.52 mu m, 2.11 at 0.55 mu m, and 6.69 at 0.58 mu m, respectively. The failure was examined by X-ray photoelectron spectroscopy and transmission electron microscopy, which was attributed to the oxidation of Ni inner electrode that could create oxygen vacancies and increase electric conduction of the ceramic capacitors. (C) 2010 Elsevier B.V. All rights reserved.