Starch films plasticized with glycerol(weight ratio of starch to glycerol=10:3) were prepared by casting and were subsequently coated with polyurethane(PU) synthesized from castor oil and tolylene diisocyanate(TDI). The coating process was performed using a motor-driven dip-coater and thickness of coated layers was 5~23${mu}{ extrm}{m}$. The coated layers were stabilized by covalent bonds between excess isocyanate groups of PU and hydroxyl groups of starch films, proved by FT-IR, DSC, and SEM. The effects of PU-coating on water-sorption behavior and mechanical properties of the films were investigated. The flexibility of uncoated films increased with relative humidity(R.H.), however, mechanical properties such as elastic modulus and tensile strength decreased significantly, due to the plasticizing effects of glycerol and water at ambient temperature. Overall absorption/desorption behavior of the coated films under different R.H. conditions was similar to that of uncoated film, whereas sorption rate and amount of sorbed water decreased with increasing coating thickness. In addition, elastic modulus and tensile strength of the coated films increased with thickness of the coated layers in the R.H. ranges investigated when compared with the uncoated films. Therefore, it could be concluded that the water-proof and mechanical properties of starch films were improved by PU-coating.