Solvent cleaning and oxygen plasma treatment were carried out on the indium-tin oxide (ITO) anodes for polymer light-emitting diodes (PLEDs). The influences of different treatment methods on the surface properties of ITO anodes were investigated using different standard surface characterization techniques, i.e., X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and contact angle. Based on the measurements of contact angles, the surface energies of the ITO anodes were also calculated using the harmonic-mean method. We observe that the surface treatments decrease the carbon content, increase the oxygen content, reduce the surface roughness and enhance the wettability, and thereby improve the surface properties of ITO anodes. With respect to the solvent cleaning, the oxygen plasma treatment is more efficient since it brings about a better improvement in the ITO surface properties due to the contaminants can be removed effectively from the ITO surface. Furthermore, we studied the effect of surface treatments of the ITO anodes on the characteristics of PLEDs, in terms of the electrical and optical performance of the devices. Experimental results reveal that the surface treatments improve the electroluminescent characteristics of the PLEDs. The oxygen plasma treated device gives smaller turn-on voltage and driving voltage, higher brightness and efficiency as well as better quality of light emission compared to the solvent cleaned device. The result can be attributed to oxygen plasma treatment improves the surface properties of ITO anode and the interface characteristics of ITO/polymer.