Background and Objectives: Muscles are known as organs which can generate pain in human body. Muscle tissues more than other organs are predisposed minor tearing during activities of daily living and cause pain. Among different types of muscle pain, myofascial trigger points (MTPs) are very common. The aim of this study was to evaluate the effect of the low power laser in the treatment of the MTPs of trapezius muscles.Material and Methods: A double blinded clinical trial conducted on a consecutively selected sample of 60 patients between 4 may 2007 and 5 August 2008, in the Fattemieh physiotherapy clinic of Rafsanjan. Respondents who were all suffering from MTPs of trapezius muscles, were divided into two groups, randomly (30 in each group). Treatment methods were included low power laser and placcbo. Intensity of the pain, improvement of movement, the tense band, reduction of the redicular pain and the sensitivity of the tense band were assessed in 1st, 5th, 10th sessions and also 10 days after the treatment. Pain intensity was evaluated by visual Analogue scale (VAS).Results: T he results of this study showed that low power laser can be more effective than placcebo in relief of the local pain of the MTPs (p<0.0001). In 10th session, chi-squire test showed that low power laser in comparison with placcebo, can be effective in removing the tense band of the muscle, reducation of redicular pain, improvement of the movements and reduction of the sensitivity of tense band (p<0.0001).Conclusion: T his study can be concluded that in the 10th session, low power laser can be effective on the relief of local pain, redicular pain, removing of muscle tense band, improving of movement and reducion of the sensitivity along tense bend of the MTPs in the trapezius muscles.