A series of aqueous polyurethane dispersions containing pendant carboxylate anion as the hydrophilic were synthesized, and the particle size and the mechanical properties of the dispersions were measured. The effects of the molar ratio of NCO/OH groups or the other functional groups on the chemical structure and the properties were discussed. The particle size mainly depended on the molar ration of the functional groups. The -NH groups in urethane and urea linkage formed hydrogen bonding with the carbonyl group (C=O) of other urethane group in the hard segments or ether group (C-O-C) in the soft segments. Fourier transform infrared (FTIR) analysis showed that the higher molar ratio of NCO/OH concentrated the -NH group, leading to the hydrogen bonding which resulted in the increasing mechanical strength of the polyurethane. The tensile testing and dynamic mechanical analysis measurement revealed that the pendant carboxylate anion made the modulus and the stiffness of the polyurethane increase.