An underwater plasma source with a porous glass membrane which functioned in bubble supply and formation of micro-discharges was proposed. We first observed behaviour of bubbles and discharge emission on the surface of the membrane using a high-speed camera and a fiber spectroscope. Additionally, decomposition of acetic acid using an underwater plasma was evaluated. Compared with an ozone bubbling method, there was remarkable decrease in the acetic acid concentration. The results showed that our proposed underwater plasma could generate radicals and effectively decompose recalcitrant organic matter.