The structures and energies of anhydrate and hydrate (hydrate rate: h of 1) states of l-alanine (LA), glycine (G), l-proline (LP), and N-methyl glycine (MG) pentamers were calculated by quantum chemical calculation using B3LYP/6-31G(d,p), for four kinds of conformers (-extended: phi/=t-/t+, PPII: g-/t+, PPII-like: g-/g+, and -helix: g-/g-). In LA and G, which have imino proton (NH), three conformation types of -extended, PPII-like, and -helix were obtained, and water molecules were mainly inserted between intra-molecular hydrogen bond of COHN in PPII-like and -helix and attached to CO group in -extended. In LA and G, PPII-like conformers were the most stable in the anhydrate and hydrate states, and the result for LA was different from some experimental and theoretical results reported by other works reporting that the main stable conformation of alanine oligopeptide was PPII. On the other hand, in LP and MG, which have no imino proton, two conformers (PPII and PPII-like) and three conformers (PPII, PPII-like, and -helix) were obtained, respectively. PPII conformers were the most stable in the anhydrate and hydrate states, and the result for LP supported the reported experimental results that the main conformation of proline oligomer was PPII. It was found that the formation pattern and stability of conformation of oligopeptide were strongly dominated by the presence/absence of intra-molecular hydrogen bonding of COHN or the presence/absence of NH2 group in the starting amino acid.