Photodegradation of solvent-cast collagen type I films and photostabilization of collagen by vitamin E were studied. These films were exposed to polychromatic radiation from a medium-pressure mercury lamp or monochromatic radiation from the Okazaki Large Spectrograph (OLS). Changes in the molecular structure of collagen were followed by UV-visible and FTIR spectroscopic measurements. Electron spin resonance (ESR) spectroscopic measurements were also carried out to identify the reaction intermediates of photodegradation. Photoreaction from phenylalanine, which is one of the main constituents of collagen to tyrosine and the scission of peptide linkage of collagen, were confirmed. Vitamin E was found to be an efficient photostabilizer for photodegradation of collagen. Action spectra of photodegradation and photostabilization of a collagen molecule were reported. A possible mechanism for the photodegradation of the collagen and photostabilization scheme based on these experimental results are presented.