A variety of castor oil-based cationic polyurethane dispersions (PUDs) were synthesized. The effects of hard segment content on the properties of PUDs and the resulting polyurethane films were investigated by mastersizer, Fourier transform infrared (IR), thermal gravimetric analysis (TGA) and stress-strain test. As the hard segment content increased from 37.1 to 45.6 wt %, the particle size of the PUDs increased from 10 to 190 nm. IR analysis showed that hydrogen bonding between hard segments enhanced with the increase of hard segment content. For this enhancement the films had an increased tensile strength from 2.0 to 9.4 MPa and an improved glass transition temperature (Tg) from 48oC to 75oC. TGA showed the films were thermally stable up to 200oC and had biphasic degradation at around 330oC and 440oC. The versatile castor oil-based cationic waterborne polyurethanes are promising in applications as adhesives and coatings.