Abstract: The cyclocondensation reaction of N-(4-methyl-6-oxo-1,6-dihydropyrimidin-2-yl)guanidine with benzaldehyde was found to result in the formation of 2-amino-4-phenyl-4,6-dihydro-1(3)(9)H-pyrimido[1,2-a][1,3,5]triazin-6-one. The reaction proceeded chemo- and regioselectively affording the 1,3,5-triazine ring closure at nitrogen atom adjacent to carbonyl group. The structure of 2-amino-4-phenyl-4,6-dihydro-1(3)(9)H-pyrimido[1,2-a][1,3,5]triazin-6-one was supported by 1H,13C NMR and 2D NOESY spectral data. From the experimental data, 3H- tautomeric form seemed to be predominant in DMSO-d6 solution. The relative energies of the tautomers were estimated using calculations at different levels of theory (HF/6-311G**, B3LYP/6-311++G** and MP2/6-311++G**). Both the experimental and theoretical results excluded 6-hydroxy tautomer from the equilibrium. 2-Amino-4-phenyl-4,6-dihydro-1H-pyrimido[1,2-a][1,3,5]triazin-6-one was calculated to be the energetically preferred tautomeric form in gas phase.