The structures and conformational energies of methylamine and ethylenediamine have been studied extensively with ab initio molecular orbital theory. For methylamine, there are good linear relationships among the C-N bond length, cosine of the H-C-N-H dihedral angle, and conformational energy. For ethylenediamine which has numerous multiminima, we studied the stereoelectronic effect, steric effect, intramolecular hydrogen bonding, repulsion between hydrogens, and repulsion between lone-pair electrons. The major factor determining the conformational stabilities of the multiminima is found to be the stereoelectronic effect. The partial hydrogen bonding, though weak, contributes to the structural stability so that two isoenergetic gauche conformers become the lowest energy structures.