The mechanism of leakage current of porous organosilicate glass (POSG) with O2 plasma ashing is investigated. The O2 plasma ashing often deteriorates the POSG film, causing moisture uptake. We find that the leakage-current mechanism of POSG transforms from Schottky emission into ionic conduction after O2 plasma treatment. The mobile ions (H+, OH-) supported by absorbing moisture move easily when an external electric field is applied to POSG film, which leads to the ionic conduction leakage current across the moisture-absorbing POSG film. Hence, the leakage current density is drastically increased about four to five orders of magnitude compared with untreated POSG film.