There are several tautomers of nucleobases. Schematic study on the tautomerization of guanine (G) has been analysed from certain reaction pathway. The differences of energies of several tautomers are found very small. Tautomerization ofGhas been analysed by usingB3LYP/6-31+G(d,p) calculations. The formation of tautomers due to H shifting fromcompatible H-bonding sites is shown in this tautomerization pathway. Transformation of normalGto tautomer cisG1 is foundmost favourable and other tautomers are formed at narrowenergy gaps. The activation energies of forming transG4 and 1cisG4 tautomers is small compared to those of other tautomers having large differences of energies fromnormalG. The energy difference of cisG1 tautomer fromnormalGis small(0.720 kcal/mol).Hence the existence of this particular tautomer might be more feasible than others which also agree with the experimental reports.