Objective To assess the in vitro effect of high intensity narrow-band blue light on bacteria isolated from patients with acne vulgaris (AV). Methods Bacteria were isolated by culture from the acne lesions of 67 patients with mild to moderate AV. Selected bacterial strains were chosen for irradiation with the blue light. The morphology and number of colony formation units (CFUs) as well as the ultrastructure of the bacteria were observed before and after the irradiation. Results Propionibacterium acnes (P. acnes) (32.4%), P. granulosum (6.9%), Staphylococcus epidermidis (S. epidermidis) (35.3%), S. aureus (4.9%), S. heamolyticus (7.8%), S.cohnii (1.0%), Corynebacterium spp. (9.8%), Klebsiella pneumoniae (1.0%) and B group Streptococci (1.0%) were isolated from the lesions. In vitro investigation revealed that the irradiation significantly reduced the number of CFUs of P. acne and S. epidermidis. Conclusion High intensity narrow-band blue light is effective against bacteria isolated from AV lesions.