With a miniaturization of electrical devices, the gap between electrode becomes lower and lower. In order to optimize the insulation design, the understanding of breakdown phenomenon across micrometer-scale gap is essential. In this study, breakdown characteristics across micrometer-scale gap was investigated based on experiment and numerical simulation. The result shows that the breakdown voltage under positive voltage application is 1.5 times higher than the one under the negative voltage application. The polarity effect can be explained by the source of initial electrons. Under negative voltage application, the initial electrons are emitted from the cathode surface based on Fowler-Nordheim equation. On the other hand, under positive voltage application, the insulator plays an important role on breakdown voltage.