In the thin film transistor fabrication process, tin doped indium oxide (ITO) or zinc doped indium oxide (IZO) film can be easily exposed to hydrogen-containing plasma during the deposition of silicon nitride (SiNx) film. By this exposure, ITO or IZO can be easily reduced into its corresponding metallic element such as indium, which degrades the optical transmittance and the conductivity. In this study, SiNx was deposited onto ITO or IZO film, and the oxygen reduction of ITO or IZO during PECVD SiNx deposition was analyzed to clarify this phenomenon. The oxygen reduction during PECVD SiNx deposition is mainly induced by decomposed NH3 gas. However, the progress of ITO reduction is different from that of IZO reduction, due to the different atomic composition of In2O3 in the composite and the different critical temperature of reduction initiation between indium oxide, tin oxide, and zinc oxide.