It has been widely known that the cigarettes contain many harmful substances, but the public may not be aware of detailed biological mechanism. Those carcinogens, including Benzo(a)pyrene (abbreviation as Bap), can potentially lead to various human cancers. When people smoke or second-hand smoke, Bap will enter their bodies through respiratory system, especially be absorbed by the lung cells. Then, Bap defuses into the cell nuclei and inserts into their DNA. Some of Bap derivatives can also form covalent linkage with nucleobase guanine. Both of these two interactions can distort DNA structure and cause mismatching in base pairing during DNA duplication process. And these mutations will possibly end up with human tumor growth and cancer generation.