BACKGROUND AND OBJECTIVE: Recurrent oral aphthous ulcers (Recurrent Aphthous Stomatitis = RAS) is a common mucosal lesions occur in 20% of the population and majority of the current treatments for recurrent aphthous stomatitis (ras) are aimed to minimizing pain. The aim of this study was to determine whether laser co2 has this analgesic effect.METHODS: A randomized single blind trial was conducted in 43 patents with ras. The subjects were divided into 3 groups: namely, co2 laser group, group with conventional treatment and placebo group. The co2 laser device was operated 3 w power in continuous mode for 5-8 seconds. To measure the pain, VAS scale was used and patients were evaluated for pain and size of the ulcers within before treatment, and days of 1, 4, and 7. FINDINGS: The average intensity of pain was 5.94±2.41, before the intervention and reached to 0.39±1.28 on the seventh day. The pain reduction in co2 laser treatment group is more than the placebo group and conventional therapy group (p=0.001). The average size of the ulcers was 4.41±2.21 mm before the intervention and decreased to 0.6±1.19 on the seventh day, but there were no statistically significant differences in the size of ulcers between groups (p>0.05).CONCLUSION: It seems that the use of laser CO2 has favorable results in reducing pain in patients with RAS.