The effect of nanosized MgO loading on the corrosion barrier characteristics of polyurethane (PU) films on mild steel has been investigated. It was found that small amount of MgO loading even as low as 0.03 vol% significantly improved the corrosion retardation ability of PU films. Both the enhanced adhesion strength and the formation of chemically strong -NCO radicals between nano-MgO and PU matrix seem to be responsible for the enhancement.