Castor oil (CO)-based reactive diluent for UV-curable urethane acrylate oligomers was synthesized by the reaction of a CO with diethanolamine and the resultant product was subsequently reacted with allyl chloroformate. The reactive diluent was characterized by FTIR and ¹H NMR techniques. This reactive diluent was incorporated in a urethane acrylate oligomer and photoinitiator at different weight fractions varying from 5 to 25 wt% for UV curing. The effects of reactive diluent concentrations on the viscosity of the formulations along with mechanical, chemical, and optical properties of UV-curable coatings were studied. Furthermore, the cured films were evaluated for glass transition temperature (T _g) determined by differential scanning calorimetry and thermal stability by thermogravimetric analysis. Gel content and water absorption of UV-cured films were evaluated.