Abstract: AM1 calculations were performed on the adduction reactions between malondialdehyde (MDA) and DNA bases. The dynamic and thermal characteristics of the reactions were analyzed from the viewpoint of geometry ical structure, electric charges distribution, and energy change. It was concluded that additive reactions, initiated by the nucleophilic atoms in DNA bases attacking the carbonyl in MDA, take place in three steps with the results of DNA cross-link. The formation of TS1, with large activation energy, is the determinant step. The total reactions are exothermal, therefore are easy to occur thermo-dynamically. Electrons transfer from bases to MDA during the reaction progress, while the protonation of carbonyl oxygen facilitates this process. The formation of crosslinking between MDA and DNA bases can cause the coding errors during DNA replica. That may be the essential origin of the carcinogenesis of MDA.