Low level light therapy (LLLT) is known to accelerate the process of wound healing, bone and cartilage formation, and to decrease tissue necrosis and inflammation. It also can be applicable to acute and chronic injury or degenerative disease. Despite forty- plus years of accumulating exprerience of the clinical efficacy of LLLT, their molecular biologic evidence is not fully elucidated. The aim of this review is to explore the role of the ROS system and its molecular biologic mechanism in related with inflammatory response, anti-apoptosis and angiogenesis during LLLT. We suggest that activation of ROS system explains many (if not most) of the observed response of cells to LLLT in vitro, and is likely to play a pivotal role in the response of animals and patients to LLLT for both experimental and clinical indications and diseases.