AbstractDiethanolamine was polymerized onto glass substrates to obtain plasma polymerized diethanolamine (PPDEOA) thin films by a plasma reactor consisting of two parallel plates through AC plasma polymerization. The surface morphology of PPDEOA thin films of various thicknesses taken at different magnifications was observed to be mosaic‐like structure. The films were uniform, defect‐free, and pinhole‐free. The average roughness and root mean square roughness values are 0.706–0.997 and 1.0–1.3 nm, respectively, as observed by atomic force microscope. The structural difference between the monomer and the PPDEOA was analyzed by Fourier transform infrared spectroscopy. The direct and indirect band gaps of PPDEOA thin films were found to be 3.15–2.83 and 1.98–1.27 eV, respectively. Urbach energy, steepness parameter, refractive index, and extinction coefficient were also evaluated. All optical parameters of the PPDEOA thin films determined in this study varied with the change in thickness.