The reaction mechanism of guanine with trans-4-hydroxyl-2-nonenal (4-HNE) and the mutagenic mechanism induced by adducts have been theoretically predicted at a molecular level from the energy point of view. 4-HNE directly reacts with guanine via three steps, yielding eventually four main diastereoisomers: trans-4-HNE-dG adducts. A concerted six-atom-centered transition state is proposed for the first step, while the last two steps are involved in four-membered-ring transition states. The third step is the rate-determining step. The studies of base pairing properties of trans-4-HNE-dG adducts with A, T, C, A*, and T* together with the relationship between the mutation and structure of trans-4-HNE-dG indicate that syn- and anti-conformations of trans-4-HNE-dG around the glycosidic bond are favorable for pairing with A* and T*, respectively, in the parental generation. As a result, the GC -> CG or GC -> TA mutation may be generated from the syn-4-HNE-dGA* during replication. Nevertheless, anti-4-HNE-dGT* creates GC -> TA mutation or nonmutagenesis. Moreover, syn-4-HNE-dGA* has a slightly higher probability to be generated than anti-4-HNE-dGT* in the parental generation; therefore, the GC to TA transversion is predominant among the mutations. In addition, no correlation between the mutations and the stereochemistry of C6 and C8 of trans-4-HNE-dG adducts was found in this work. Our mutational results have interpreted well a part of the discrete experimental observations, but the mutagenic process itself has not previously been characterized, through either computation or experiment.