A planar type of inductively coupled thermal plasma (ICTP) with coil current modulation was adopted for large-area rapid surface oxidation of a substrate. The planar ICTP has been developed by the authors for large-area surface modification processing. In addition, coil current modulation was used to control the temperature and chemical reaction fields in the planar ICTP. Firstly, a fundamental study of the operation of the planar modulated ICTP with a substrate was carried out by measuring its electrical properties and the visible light emission in this work. Secondly, spectroscopic observations were carried out to investigate the effect of the coil current modulation on the radiation intensity distribution of spectral lines from the planar ICTP on the substrate. Thirdly, surface oxidation tests were made for Si substrates by irradiation of the planar modulated ICTP at different modulation frequencies and different duty factors. Oxide layer thickness distribution fabricated on the substrate was measured to study the lateral uniformity of the oxidation processing by the modulated planar ICTP. Finally, it was found that adoption of the coil modulation can improve the uniformity of the oxidation processing by the planar modulated ICTP.