Abstract: An anodized carbon fiber tow was continuously sized using aqueous polyurethane to improve the interfacial properties of carbon fiber-reinforced polyurethane composites. These were investigated by interlaminar shear strength (ILSS) tests, elemental and functional group analysis, thermal gravimetric analysis, and differential scanning calorimetry. Results show that the polyurethane sizing agent significantly improves the interfacial properties of the composites. The ILSS of the sized carbon fiber-reinforced composite is increased by 17.5% (from 39.5 to 46.4 MPa) compared to that of the oxidized carbon fiber-reinforced counterpart. Treating the sized carbon fiber-reinforced composite at 170 °C further increased the ILSS by 9.5% to 50.8 MPa. It is considered that the sizing agent interacts with oxygen-containing functional groups on the oxidized carbon fiber surface to form hydrogen bonds with the resin matrix. Upon heating at 170 °C, blocking groups in the sizing agent are unblocked to expose the isocyanate roots that react with the carbamate of the matrix to generate allophanate. It is concluded that a polyurethane sizing agent is suitable for improving the interfacial properties of carbon fiber-reinforced polyurethane resin composites, and that heating after curing further improves these properties.