The interconversion process from formamide to its enol form formamidic acid (FM --> FA) can be regarded as a model for tautomerization of larger nucleic acid bases 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)(n) and the enol form formamidic acid-(H2O) (n = 0-3), by means of B3LYP exchange and correlation functions. In the vicinity of formamide (FM) and formamidic acid (FA), three different regions are considered. Water in two of them can protect formamide from tautomerizing, while in the third one works oppositely. The protective and assistant effects of water molecules in the same and different regions have been discussed in detail with the number of water molecules increasing. The calculated results offer us a new insight into the structural tautomer interconversion of FM.