The effects on structural variation and relative stability caused by hydration of alanine dipeptide in the C7eq, C7ax, C5, alpha(R) and P(II) conformations were investigated by the method of the SUMSL (secant-type unconstrained minimization solver) energy-minimization technique, using the empirical potential energy function of the solute and the ST2 water model. The binding sites of water molecules in the solute were selected from spherical interaction energy maps. As a function of the number of added water molecules, we traced the structural variations mainly in the (phi, psi) torsion angle space, and calculated the total internal energies of each solute-water system to evaluate the relative probabilities between those five conformers. The results indicate that the conformers having no intramolecular hydrogen bonds, such as alpha(R), C5 and P(II), shows larger structural variations than the C7ax and C7eq conformers which have intramolecular hydrogen bonds. The torsion angle, psi, fluctuated more than phi for all the examined conformers. The energetically dominant conformation is shifted from C7eq to alpha(R) as the hydration progresses.