Abstract: The interconversion process between keto and enol forms of formamide (FM→FA) can be regarded as a model for tautomerization of larger nucleic acid base, which may be responsible for the spontaneous point mutation in DNA. The present paper describes a study of structural tautomer interconversion and the relative stabilizing influences of water for formamide-H2O and the enol form formamidic acid-H2O in aqueous solution. The solvent effects were considered by explicit water molecule within SCRF framework. In the vicinity of formamide (FM) and formamidic acid (FA), three different regions are considered, full geometry optimizations of these complexes were carried out at B3LYP/6-311++G** using self-consistent isodensity polarized continuum model (SCIPCM). The calculated results indicate that water in two of them can protect formamide from tautomerizing, while in the third one works oppositely, exactly the same as the situations in gas phase. However, comparing with the transition in gas phase, the protective and assistive effects induced by the explicit water are greatly altered. The calculated results shed more light on the water′s influence on the proton transfer and offer a new insight into the structural tautomer interconversion of FM.