A method of treating a surface of a substrate using modified urushiol derived from Toxicodendron vernicifluum is provided. More particularly, the reactivity of a hydroxyl group of urushiol extracted from fresh Toxicodendron vernicifluum is removed before the lacquer is used as a UV coating agent for a substrate such as a steel sheet. Therefore, the substrate may have high antibacterial activity, and excellent appearance and functionalities such as far-infrared radiation, blocking of electromagnetic waves, enhanced corrosion resistance, high crosslinking speed when a low content of a photoinitiator is used, excellent surface gloss and high scratch resistance.