Temporomandibular disorders (TMD) are a set of conditions concerning the temporomandibular joints and the masticatory muscles and involve musculoskeletal pain, disturbances in the mandibular movement patterns and function. The aim of this study was to explore the efficiency of two modalities of physical treatment of chronic myalgia of masseter muscle and the role of psychological factors in success of treatment. The hypothesis is that both therapeutic modalities significantly reduce pain and disturbance of the mandibular function, where laser is more efficient. It is expected that is the most powerful moderator of success of treatment is the health competence, i.e. the degree to which an individual feels capable of effectively managing his or her health outcomes. Materials and methods: The research was randomized controlled trial. The instrument Diagnostic Criteria for TMD was used to select subjects and monitor treatment outcome. 54 subjects with masseter myalgia of more than 3 months in duration were invited to participated and 42 cases were available at the end and analysed. Age range was 16-67 years (median 33, interquartile range 25-53) and there were 62% females. Efficiency of two treatments modalities in subjects with diagnosed masseter myalgia were tested: 1) self-management (counselling about the problem, parafunctional behaviour identification, monitoring and avoidance, home self-massages of painful muscle points and thermal therapy with moist warm wraps) and 2) biostimulatory diode laser for one month. Each group had 21 participants. During treatment participants did not use analgesics. Subjects were monitored one months after the start of treatment and three months after that. Results: Laser and self-management reduce symptoms of chronic myalgia of the massager: self-reported limitation of jaw function (chewing, mobility, verbal and non-verbal communication) in the short term and reduce pain intensities (p<0.05). Characteristic pain intensity decreased significantly in both treatment groups, with larger effect size in the laser group than self-management group (p<0.001; η2 = 0.954 and 0.453). Disability score also decreased significantly in both groups, more significantly in the laser-treated group (p≤0.035; η2=0.751 and 0.154). Self-management group demonstrated reduction of effect after three months, which was not the case after laser therapy.Mouth opening, laterotrusion movements and protrusions increased in both groups after therapy. The difference was always statistically significant in the laser-treated group with large effect size (p<0.001; η2=0.786-0.861). In self-management group the difference was significant only in maximum assisted opening with medium effect size (p=0.014; η2=0.217). There were no significant differences between groups at any time points. Psychological characteristics significantly affect the ffectiveness of treatment, more in reducing the limitation of jaw function than in reducing the intensity of pain. Poor treatment efficacy was not influenced by the health competence, but with somatosensory amplification and catastrophizing (p <0.05). Conclusion: Both self-management and laser are effective in treatment of chronic masseter myalgia but the laser has a stronger and longer effect. The cognitive distortion, seen in tendency to exaggerate and misinterpret bodily sensations, diminishes the effectiveness of treatment.