A series of waterborne polyurethane dispersions (PUDs) were synthesized using castor oil, polypropylene glycol, toluene diisocyanate, and (2E)-4-(2,3-dihydroxypropoxy)-4-oxobut-2-enoic acid [glycol semi-ester (GSE)]. These PUDs can be crosslinked spontaneously upon drying, without extra additives or processing steps. Moreover, the rheological properties, particle size, and morphology of PUDs were examined with stress-controlled rheometer, light scattering ultrafine particle analyzer, and transmission electron microscopy. Results reveal that the particle size of PUDs mainly depends on the concentrations of castor oil, GSE, and polymer. With the increase of GSE concentration and the decrease of castor oil and polymer concentration, the particle size decreases. This can be ascribed to the decrease of the particle size of the polymer dispersion, because of the decrease of the relative size of water layer to total particle size. Furthermore, increased amount of castor oil, GSE, and polymer concentrations results in an increased viscosity of PUDs because of the increase of effective volume for the dispersed phase. POLYM. ENG. SCI., 49:162-167, 2009. (C) 2008 Society of Plastics Engineers