BACKGROUND Light-based therapies and lasers have been proposed for the treatment of acne vulgaris but the efficacy and application periods of 532-nm KTP laser treatment are not clear. OBJECTIVE To evaluate the efficacy and safety of 532-nm KTP laser and compare the effects of once and twice weekly applications in the treatment of mild to moderate acne vulgaris. METHODS Totally 38 patients were treated once weekly and twice weekly in group I and in group II respectively. One half of the face of each patient was treated with 532-nm KTP and the other half was remained as untreated. Patients were evaluated at the beginning, one and four weeks after the last treatment session with Michaëlsson acne severity grading score (MASS). RESULTS Statistically significant improvement was found at second control (p = 0.005) in group I, and at the first (p = 0.004), and second (p < 0.001) controls in group II for treated sides. For both groups, changes of MASS were insignificant for untreated sites. Improvement of MASS of treated sides was not statistically significant between two treatment groups for both controls. CONCLUSION 532-nm KTP laser treatment may be an alternative method in selected acne vulgaris patients. No significant difference was noted between once and twice weekly applications.