Monoethanolamine is a model molecule for biological systems and widespread ligand in coordination chemistry; it has a large number of isomers, of which gauche and trans are the most stable in gas and crystalline phases, respectively. The work is devoted to non-empirical study of conformational transformations of different gauche and trans isomers of individual monoethanolamine molecule. These transformations was shown to be determined by the features of formation and cleavage of intramolecular hydrogen bonds O-H⋯N and N-H⋯O and the internal rotation of substituted methyl groups. Assessments were obtained for the free energies of conformers and activation energies of their mutual transformations.