Application of protective coatings on metals may involve a thermal treatment process. In this study, the effect of thermal treatment up to 200?C on the corrosion protection was investigated for nanocomposite films composed of mussel adhesive protein (MAP), CeO2 nanoparticles and Na2HPO4 deposited on carbon steel. The morphology and microstructure of the pre-formed nanocomposite film were characterized by scanning electron microscopy/energy dispersive spectroscopy and atomic force microscopy (AFM). The changes in the chemical structure of the nanocomposite film due to the thermal treatment were investigated by infrared reflection absorption spectroscopy. The corrosion protection of the unheated and heated nanocomposite films on carbon steel was evaluated by electrochemical impedance spectroscopy and details of the corrosion process were elucidated by in-situ AFM measurements in 0.1 M NaCl solution. The results show a certain increase in the corrosion protection with time of the nanocomposite film for carbon steel. The analyses reveal that thermal treatment leads to a reduction of water molecules in the nanocomposite film, and an enhanced cross-linking and cohesion of the film due to oxidation of catechols to o-quinones. As a result, the film becomes more compact and gives improved corrosion protection for carbon steel.