Optical emission spectroscopy of the rf thermal plasma evaporated YBa2Cu3O7−x has been investigated. The deposited films with the nearly stoichiometric composition could be obtained by using Y-rich powder. From the optical emission spectra, O, Y, Y+, Ba, Ba+, Cu, and YO as the emitting species were observed. The excitation temperature was lower for Y+ than for the other dissociated elements. The relative intensity for YO of a suboxide became larger with a lower plasma torch. It is thought that the deposited film that is poor in Y results from YO sticking to the substrate rather than Y or Y+.