The N-H stretching vibration in a beta-peptide model compound, N-ethylpropionamide (NEPA), was characterized by one-dimensional infrared (1D IR) and two-dimensional (2D) IR experiments and ab initio anharmonic frequency computations. A narrowband pump-broadband probe 2D IR method was applied to selectively probe a subensemble of the N-H stretching vibrations from a mixture of different NEPA molecular aggregates that were formed via an intermolecular hydrogen bond. Vibrational lifetime and anharmonicity were found to be sensitive to the aggregation ensembles. In particular, diagonal anharmonicities were observed experimentally and confirmed computationally to be smaller for NEPA trimer than for dimer, which was explained by the presence of non-negligible off-diagonal anharmonicities in coupled N-H stretching modes.