The monosaccharides such as D--glucose and D--idose sugars differ on the hydroxyl group's orientation at the C2-C4 positions. Hydroxyl groups such as OH2, OH3, and OH4 in -idose are axial, while all (OHs) groups in -glucose are equatorial. The density functional theory based on the B3LYP/6-31G(*) level results show that -glucose is more stable than in -idose. AIM analysis indicates the existence of intra-molecular hydrogen bond so that -glucose displays regular intra-molecular hydrogen bond, while -idose displays bifurcated acceptor intra-molecular hydrogen bond. The NBO analysis has been confirmed that the formation of intra-molecular hydrogen bonds in these two sugars.