Statement of problem: Recurrent aphthous stomatitis is one of the most common ulcerative lesions of the oral cavity, but up to now no cure has been recognized for it. In some studies low level lasers had good efficacy in reducing pain intensity and healing time in recurrent aphthous stomatitis but in some other studies the results were controversial.Purpose: The aim of this study was to evaluate the efficiency of LLLT in treatment of RAS as compared with the placebo effect.Materials and Methods:  In this double blind clinical trial, 24 patients with minor RAS who didn't use any drugs for it or didn't have any other related systemic disease to RAS were selected and randomly divided into 2 groups of 12 cases and 12 controls. In the case group, laser was irradiated but for the control group, the procedure was similar but laser wasn't irradiated to lesions. In this research, diode laser (660 nm, 3 J/cm2, continuous) was applied. The pain intensity (VAS), the time of pain relief and healing time were evaluated.  The data were analyzed by T-test, pair-T-test and covariance.Results: This study showed that healing time was 5±1.41 days in the laser group and 8.25±0.96 days in the placebo group. Difference between the two groups was significant statistically ( p <0.001). According to covariance analysis, the pain difference at the end between the two groups by considering the pain difference before treatment was significant ( p =0.009).Conclusion: According to the results of this study, low level laser can decrease the healing time, pain intensity and the time of pain relief in patients with aphtae.Key words: Aphthous stomatitis, low level laser therapy, healing, pain