Zebularine, 5-aza-cytosine and 5-aza-5,6-dihydro-cytosine are structurally similar to cytosine, but their biological functions are rather different. Cytosine can be methylated which is a gene lesion that can cause human disease. On the contrary, zebularine and 5-aza-cytosine are inhibitors of DNA methylation. 5-aza-5,6-dihydro-cytosine is specifically designed to induce lethal mutagenesis in HIV for its structurally variability. Here, theoretical research into their chemical properties through density functional theory is reported. Molecular hardness and molecular electronic surface potential were analysed. Compared to cytosine, the main reason for the inability of methyl addition of zebularine is the reduced nucleophilicity of C5 atom. The lack of a hydrogen atom at N5 atom in 5-aza-cytosine is responsible for the incomplete reaction of methyl transfer. Variability of 5-aza-5,6-dihydro-cytosine is responsible for the mutagenesis treatment by paring with guanine or adenine with its different tautomers. Aspect of these chemical reactivities can be accounted for the distinctive biological functions of these molecules.