Statement of problem: Hyperactive gag reflexes in some dental patients may lead to difficulties in the treatment processes. Various methods have been introduced for control of gag reflex in dentistry.Purpose: The aim of this study was to evaluate the effect of inhalation sedation for gag reflex control in patients with hyperactive gag reflexes and improving operation condition during dental treatment.  Materials and method: In this clinical trial study 14 healthy adult patients with tow times failure in dental treatment due to the hyperactive gag reflexes were selected for treatment of posterior mandibular teeth. For all patients, treatment was performed under inhalation sedation with 50% N2O and 50% O2. Severity of gag reflexes and operation conditions was evaluated and results were analyzed using Spearman, Mann Whitney and Wilcoxon tests.Results: Of 14 patients under study, 4 patients were men (28.6%) and 10 patients were women (71.4%). Gag reflex during operation was suppressed in 11 patients (77.8%) and it was active in only three patients (21.4). Dental treatment was performed easily in nine patients (64.2%), while in three patients (21.4%) it was done with difficulty and in two patients (14.3%) no treatment was performed due to hyperactive gag reflex. Complications were not observed during treatment.Conclusion: For reducing gag reflexes and improving the operations condition in patients with severe gag reflexes it is possible to use inhalation sedation (N2O and O2) which is easy to use and safe. In case, this method is not sufficient, treatment should be performed under general anesthesia.Key words: Gag reflex, Inhalation sedation, Dental treatment