In the present work, the homopolymer built from the free radical polymerization of methacrylic monomer incorporating an azobenzene side-group has been synthesized and structurally characterized. The optical properties such as refractive index, extinction coefficient, absorption coefficient and optical energy band gap of poly[4-methacryloxy-(4-carboxy)-azobenzene] thin film prepared by high vacuum sublimation method were determined using spectroscopic ellipsometry combined with transmittance measurements.We found that in the spectrum of extinction coefficient there are absorption bands, which are assigned as the n-pi* and pi-pi* electronic transitions of the azo compound.