Photosensitized protein damage is an important process in phototoxicity and the mechanism of photodynamic therapy. Fluorometry could be used for the evaluation of the protein photo-damaging activity of a photosensitizer. In this study, we designed a procedure for the analysis of protein damage by fluorometry using a water-soluble protein, human serum albumin (HSA). HSA has one tryptophan residue, which is a strongly fluorescent amino acid and is easily oxidized by photochemical reaction. The oxidation of HSA results in a decrease of the fluorescence intensity of the tryptophan residue. These properties are appropriate to probe oxidative protein damage. On the basis of these properties of HSA and tryptophan, an analytical procedure for protein damage is proposed.