Abstract: Optical fiber absorption spectrophotometer equipped with 50 cm liquid core waveguide capillary cell was used for determination of plutonium in various oxidation states, i.e. Pu(Ⅲ), Pu(Ⅳ) and Pu(Ⅵ), in a mixed solution. Based on the spectrometric characteristic of Pu in the 3 oxidation states, 600, 474, 828 nm were chosen as the wavelengths for measuring the absorbances of Pu(Ⅲ), Pu(Ⅳ) and Pu(Ⅵ) respectively. To eliminate deviations due to drift of baselines in measurements of absorbances, values of absorbances were measured at the reference wavelengths at 640, 492, 850 nm, which were selected from the trough of the respective measuring wavelengths, and substracted from their values of absorbance of Pu(Ⅲ), Pu(Ⅳ) and Pu(Ⅵ) respectively, to obtain their valid values of absorbance. Simultaneously, standard curves and valid values of molar absorptivities were determined with Pu standard solutions of single oxidation state by measuring the absorbances at the related wavelengths. The values obtained were substituted into the given formulas and contents of Pu(Ⅲ), Pu(Ⅳ) and Pu(Ⅵ) in the mixture were calculated. The results obtained were checked by the TTA extraction method, and consistency within the range of uncertainty between the results of the 2 methods was attained.