A spin coating was used to prepare thin films of Chitosan and its derivative. Fourier Transform Infrared (FTIR) spectroscopy was adopted to identification of chemical bonds and functional groups of chitosan and its derivative. The optical properties of thin films were investigated using the spectrophotometric measurements of the transmittance, T(λ), and reflectance, R(λ), at normal incidence of light in the wavelength range 200 - 2500 nm. The complex refractive index (n*) and complex dielectric constant (*), as well as, the nonlinear optical properties (third order nonlinear susceptibility and the nonlinear refractive index) of Chitosan and its derivative was determined and analyzed. A spin coating was used to prepare thin films of Chitosan and its derivative. Fourier Transform Infrared (FTIR) spectroscopy was adopted to identification of chemical bonds and functional groups of chitosan and its derivative. The optical properties of thin films were investigated using the spectrophotometric measurements of the transmittance, T(λ), and reflectance, R(λ), at normal incidence of light in the wavelength range 200 - 2500 nm. The complex refractive index (n*) and complex dielectric constant (*), as well as, the nonlinear optical properties (third order nonlinear susceptibility and the nonlinear refractive index) of Chitosan and its derivative was determined and analyzed.