In this study, WC alloy micro-pins were fabricated by electrochemical machining using a NaNO3 aqueous solution and bipolar pulse supply. Since the influence of the pulse conditions on micro-pin formation is still unknown, the micro-pin fabrication was carried out by changing the pulse period, the pulse width, and the power supply system. In order to understand the mechanism of machining using bipolar pulses, observations of the micro-pin surface during the process were carried out using a high-speed camera. As a result, it was found that the colour of pin surface becomes blue momentarily when the pulse voltage falls in the case of using a bipolar pulse power supply. In addition, the relationship between the pulse waveform and the blue appearance period was investigated and discussed.