Aqueous dispersion of polyurethane(PU) ionomers, based on poly(tetramethylene glycol) as soft segment, isophorone diisocyanate as diisocyanate, 1,4-butanediol as chain extender, dimethylol propionic acid as potential ionic center, triethylene tetramine as crosslinker, and triethyl amine as neutralizer, were reinforced with hydrophobic nanosilica to give waterborne PU/nanosilica composites. The reinforcing effects of nanosilica were examined in terms of mechanical, dynamic mechanical, and thermal properties together with water swell and transparency of the dispersion cast films. With the addition of nanosilica to PU, tensile modulus and strength increased, and the rubbery modulus increased in magnitude and extended to higher temperature, and these enhanced mechanical and thermal properties, together with increased swell resistance were obtained without scarifying the transparency of the films.