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A novel measurement method for determining mode power distribution in a multimode parabolic-index fiber is developed. Using this method, the mode power distributions are obtained in terms of the principal mode number by numerically processing the measured Fraunhofer diffraction patterns of the near-field patterns on an output fiber endface. As an example, differential mode attenuation of a multimode parabolic-index fiber is measured. It is confirmed experimentally that the method is practically applicable to the mode power distribution measurements in a parabolic-index fiber.