Density functional theory (DFT) calculations were performed to study the conformations, hydrogen-bonding effects, and the stabilities of different methyl group orientations of isolated trans- and cis-N-methylacetamide (NMA) and of 12 NMA-water complexes. The DFT functionals B3LYP, BLYP, and the basis set 6-311++G-(d,p) were used all through the calculations. The isolated trans-NMA structures obtained from these two DFT methods are consistent with experimental data from gas-phase electron diffraction and comparable to MP2 calculations at a much lower cost. Cooperative effects of hydrogen bonding were found in the trans-NMA . 2H(2)O complexes, in which the two water molecules attach to the N-H and C=O groups of trans-NMA, respectively. No substantial H-bonding cooperative effect was found for cis-NMA-water complexes. The trans-NMA structures with the methyl orientations Phi = Psi = 0 degrees and Phi = 180 degrees and Psi = 0 degrees are the most stable conformations of isolated trans-NMA, and the structure with Phi = 180 degrees and Psi = 0 degrees corresponds to the energy minimum state of trans-NMA in hydrated situations. The methyl group orientation of Phi = Psi = 180 degrees corresponds to the most stable structure both for gas-phase cis-NMA and the cis-NMA-water complexes.