We present a study of a resist ashing using O-2 inductively coupled plasma (ICP) at low pressure (< 10 Pa). An ashing rate of 1.5 m was obtained at 0.6 Pa by direct exposure to the plasma. However, a serious charge-up damage appeared owing to the radiation of a large amount of charged particles. On the contrary, a structure modification to restrict the charged particles could suppress the serious charge-up damage. The ashing rate dropped to below 20% of that directly exposed to the plasma. These ashing rates were proportional to the O* spectrum intensity directly above the substrate, which was strongly dependent on the pressure. The amount of the exited particles supplied to the substrate depended on the phenomena as a function of the pressure, such as a mean free path, a plasma distribution, etc. In this paper, the alternative magnetic field measurement in the plasma indicates the energy absorption decrease to the plasma at high pressure, which is an important factor of the ICP production for processing.