Abstract: We aimed to determine the features of ultraviolet and infrared spectra of the pigment brown in natural cotton fiber,analyze the effects of pH values and concentrations, diagnose reagents in the ultraviolet spectra, and identify the chemical properties of the pigment. We first purified the brown pigment for use in all experiments. The ultraviolet spectra varied with pH values and concentrations and was influenced by diagnosed reagents. However, the effects of diagnosed reagents were not the same as those of flavones. The infrared spectra showed the wave numbers 3 554, 3 477, 3 414, 3 289, 1 638, 1 618, 1 386, 1 073,617, and 478 cm-1. The chemical features and purification methods of the brown pigment were compared with those of the pigment in the seed coat of white conventional cotton, studied by Halloin in 1982, revealing similarities between the two pigments. Thus, the brown pigment in natural cotton fiber is a quinone compound from tannin oxidation at the end of fiber development.