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Proso millet is rich in protein, minerals, dietary fiber, and vitamins, and its nutritive parameters are comparable to or better than other common cereals. Additionally, the gluten-free and non-GMO properties of millet make it desirable for human food. However, the structural changes of millet protein during cooking/processing is unclear, which plays a major role of the bioavailability of millet protein. The objective of this work was to study the conformational changes of prolamin from proso millet under different conditions mainly using fluorescence spectroscopy. The prolamin was extracted and purified in 70% ethanol solution. Fluorescence spectra of prolamin in ethanol-water solution from 80% to 20% were obtained. The emission maximum of prolamin was increased with decreased water content, which can be attributed to the exposure of the hydrophobic regions in the protein to solvent. Fluorescence intensity of prolamin in aqueous solution was decreased by both guanidine hydrochloride (Gdn·HCl) and heating due to the denaturation. The emission maximum of prolamin red shifted by 10 nm after heating at 90 °C for 1 h, indicating that the tryptophans in prolamin were exposed to a more polar environment. The interaction of tannic acid with prolamin in aqueous solution containing 0.7%, 70% ethanol or 0.4 M Gdn·HCl was also studied by tryptophan fluorescence quenching. And the binding affinity of prolamin with tannic acid was decreased after partial denaturation. Findings from this study contributes to the understanding of millet protein chemistry, which may be useful in improving the bioavailability of millet protein.