High levels of active oxygen species generated by carcinogenic chemicals can cause DNA damage, which may lead to carcinogenesis. We have investigated the characteristics and mechanisms of DNA damage induced by some carcinogens. Here we show our experimental results and propose the possible mechanisms of DNA damage through a) NADH-dependent and b) manganese-dependent formation of reactive oxygen species. We also discuss the mechanism of c) DNA damage induced by nitric oxide and peroxynitrite.