The formation condition, microstructure, and growth kinetics of the WO X layer for WO X ReRAM are investigated. To understand the optimal condition for the rapid thermal oxidation process which forms the WO X layer, various annealing temperature and annealing time are systemically studied through TEM, XRD, Raman spectra analyses and electrical characterizations. The growth kinetics for WO X under RTO is found similar to the one for thermal oxidation on silicon. The electrical forming voltages of the WO X cells are also found independent from the oxide thickness, which further suggests the switching behavior of WO X ReRAM takes place at the interface but not the bulk.