Objective:This study was designed to evaluate the effect of single dose of low-level laser therapy on  pain, swelling and trismus  after impacted lower third molar surgery.  Subjects and methods: Twenty patients with bilateral impacted lower third molars on both sides were selected. They were selected from those attending the out-Patient Clinics of Oral and Maxillofacial Surgery Department, Faculty of Dental Medicine, Boys, Cairo , Al Azhar University and Sayed Jalal University Hospital.The teeth of patients were removed in two separate operations. Postoperatively, the patients received laser therapy with energy of 4J/cm2 on one side and no laser energy was applied to the other side (placebo side). Swelling, trismus, and subjective assessment of pain on a visual analog scale were evaluated and compared between the laser-treated and placebo sides. . All readings were recorded and analyzed statistically.  Results:  There was statistically significant difference in pain, swelling and  trismus between the laser treated and placebo sides ( p>0.001). However, according to the clinical outcomes, swelling and trismus were less in the laser-treated side than in the placebo side. Conclusion: A single-session LLLT with a diode laser that was performed immediately after impacted tooth extraction gave a beneficial effect for pain reduction, anti-inflammatory processes and increase the maximum mouth opening.