Orthodontists need to inform patients of different appliance alternatives, extent of expected success during the consultation stages. However they still could not predict the degree of the treatment success. Power spectral density (PSD), energy of power spectral density and area under the power spectral density signals may be an index to evaluate of the effect of these appliances. For this aim, variations in these features of anterior temporal muscle's electromyogram signals are analyzed. Because having non stationary character of EMG signals, PSD graphics are derived using AR method. The study sample consisted of 10 patients, 7 to 13 years old. These patients treated orthodontically with a Herbst which is one of the oral appliances. EMG recordings with and without the appliance in situ were obtained when the appliance was started to use. When we look at the findings the PSD values, the maximum energy of PSD and the area under the PSD signals of anterior temporal muscles' EMG during the Herbst is placed in situ are smaller than the their without Herbst in all patients.