The activation free energies of the hindered internal rotation around the C-N bond for N,N-dimethylcinnamamide, N,N-dimethylcrotonamide, and N,N-dimethylsorbinamide obtained by the NMR line-shape method are 16.5, 14.9, and 16.2 kcal/mole, respectively. The relation between these values and the structure of the compounds is discussed.