Density functional theory is used to optimize the minimum conformers of the lowest generation (G0) ethyl (CH2CH3)-terminated poly(amidoamine) (PAMAM) dendrimers with UB3LYP/6-31++G** methods in the gas phase. The obtained a, b, c, and d conformers are the most stable, and all the secondary amide groups are in the trans-position except for the a conformer. The terminal group rotation, the intramolecular hydrogen bonding, and interbranch interactions all minimize the energy of the conformers. The IR spectra of the lowest energy conformers of PAMAM-CH2CH3 are also studied.