Abstract: Micro-electrodes with gaps of 5-40 micrometers were prepared by depositing 400-nm thick Al film on insulator using the micro-electro-mechanical system(MEMS) manufacturing process. Breakdown voltages of different electrode gaps under different pulse polarity and nanosecond pulse widths were measured on test samples. Moreover, the breakdown mechanism was discussed through comparing the observed results of samples after breakdown by use of scanning electron microscope(SEM) and the results of the same samples under DC. In conclusion, the breakdown voltage increases with the gap distance under nanosecond pulse, but the breakdown electric field descends sharply while the gap distance increases. Compared to the breakdown voltage under DC, pulse breakdown voltage is not too much higher, which is different from the results in the case of macro distances. Analyses show that the mechanism of the breakdown is the streamer theory, and the anode is melted layer by layer, while the cathode is sputtered with metal material.