We have investigated thin film of Indium Tin Oxide (ITO) deposited on glass substrates by thermal annealing and by H3PO4 chemical etching in order to improve their quality for use in polymer based light-emitting diodes. The morphology of the film surface was significantly changed by both treatments as observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The sheet resistivity of the annealed sample was found to be dependent on the annealing temperature, while it was practically not affected by the chemical treatment. On the chemically treated samples, analysis of the film surface by X-ray photoelectron spectroscopy (XPS) revealed that a thin ionic phosphorus layer was formed on the ITO surface. The electrical and optical characteristics of light-emitting diodes with poly(2-methoxy,5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) or MEH-PPV as an active layer, deposited on treated and untreated ITO substrates were determined and compared. Subsequent enhancement of performance of the diodes was obtained with treated ITO substrates, proving the efficiency of the performed treatments.