The mechanism responsible for the oxidative stress due to photobiomodulation induced by 1265 nm laser is still unclear. Mitochondria are assumed to be the most probable acceptors of the 1265 nm laser irradiation. We study oxidative stress, mitochondrial potential, GSH, cell viability, DNA damage. We demonstrated that narrowband (highcoherent) and wideband lasers employed at the doses of 9.45 and 66.6-400 J/cm2, respectively, induce a dose-dependent cell death, increase ROS level, disturb mitochondrial functioning and can damage DNA. Thus, the 1265 nm lasers can affect the HCT116 cells through mitochondrial damage. Energy density increase contributes to cell damaging without heating effects.