Background: Chronic sinusitis is one of the most common chronic diseases involving different age groups. Because the nature and etiology of chronic sinusitis are not completely known, there is not any standard treatment for this disease. It has been suggested that low-level laser can be used in treating chronic sinusitis but there are limited studies about its usage. In this research, intra-oral radiation of low-level laser has been described and implemented for the first time. Suggested hypotheses about the efficacy of this type of radiation (intra-oral) in treating chronic maxillary sinusitis includes this fact that the depth of maxilla’s vestibule is also the floor of maxillary sinus and sinus discharges collect in this area because of gravity effect. Therefore, with considering suitable radiation angle, this area gets the most benefits of laser’s anti-inflammatory effects. Material and Methods: In this study, 20 patients with chronic maxillary sinusitis were included. They were assessed before and after treatment. Treatment plan was performed in 8 sessions every other days using low-level diode laser with 810 nm. Snot-22 questionnaire and rhinomanometry were used for evaluating patients. Changes of signs and symptoms were recorded in questionnaire every session and 6 months after treatment. Friedman and Wilcoxon tests were used for data analyses. In this study, P value < 0.05 was considered statistically significant. Results: All variables and all symptoms of patients were improved using intra-oral low-level laser and this improvement was statistically significant ( P value<0.05). There was also significant decrease in nasal airway resistance and significant increase in air flow ( P value<0.05). Six month after treatment completion, there was no significant difference between the results of completion and the results of 8th treatment session ( P value< 0.05). Conclusions: Using intra-oral low-level laser is a suitable way to treat patients with chronic maxillary sinusitis.