The effect of thinning dielectric layers on the insulation resistance of MLCCs with Ni-electrode has been studied with special reference to the microstructure. Thinning dielectric layers is effective to achieve large capacitance MLCCs. In this study, MLCCs which are consisted of 1.9μm dielectric layer thickness with one grain are evaluated. The insulation resistance (IR) of MLCCs with a 1.9μm active layer thickness changed from 105 to 1011 Ω·cm by reoxidation treatment. In order to explain the increase of the IR after annealing, the distribution of element was investigated using TEM-EDS and EPMA. As a result of analysis, segregated Mn in active layer has disappeared after annealing. These phenomena suggested that the IR faults were caused by the segregated phases.