The effect of rare earth (Ln = Gd, Dy, Y and Yb) and Mg ions on the microstructure and reliability of BaTiO3-based monolithic ceramic capacitors (MLCs) with Ni electrodes was investigated. The X-ray diffraction results about the lattice volume of sintered specimens suggested that Gd and Dy ions predominatly substituted into the Ba-site, Yb ions gave exclusive substitution at the Ti-site, while Y ions occupied either the Ba- and Ti-site. The reliability of ceramic capacitors was increasing with increasing the ionic radius of the rare earths in this study. The nonlinearity coefficient() etsimated from the leakage currents and the lifetime measured from the highly accelerated lifetime testing (HALT) showed a negative correlation which was observed only from Dy and Y ions doped specimens. The quantity of Ln and Mg in the grains tended to increase with increasing the Ln ionic radius. In order to improve the reliability and the insulation property of BaTiO3 based MLCs with Ni electrodes, it is important that acceptor ions at the Ti-site compensate donor ions which are rare earth ions at the Ba-site, so the overall quantity of the dopants required for the charge compensation with acceptor and donor ions increases accordingly.