In this study, 50 patients who had inclusion criteria were selected from patients suffering mild to average OSAS who referred to sleep clinic of Mashhad Ibn-Sina hospital. After explaining the objectives of the research and obtaining patients conscious consent to cooperate, they were included in the study. Patients were divided into two intervention and control groups using blocking random sampling. All patients received initial examination and medical records including photography, radiography, and waxing records. Then impression from mandible and maxilla was done with Alginate material from Materials-Kohler-Germany. In intervention group a record of jaws relation was taken while the patient was protruding lower jaw and in the next step, Essix clear device was made by Biostar machine according to patient's cast in the laboratory. Afterward, the Mandibular Advancement Splint was prepared based on the record of jaws. In order to prepare oral device, plate 3A and cold cure acrylic resin Acropars_Marlik-Iran were used. Control group also used placebo (oral device without jaw protrusion). Then for each patient in intervention group, pharyngeal MRI and pharyngeal volumetric were done twice, immediately with and without device. Since no pharyngeal anatomic changes happened by placebo, MRI was not performed in control group. After using devices by patients, daily sleepiness and sleep quality were evaluated by questionnaires at the beginning and one month after using the device. At last the results, MRI volumetric and possible side effects were compared.