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Fluctuation components on a backward Rayleigh scattered signal measures by the polarization optical time domain reflectometer (POTDR) have been investigated in detail by means of the least squares method and the power spectra. As a result, it is revealed that the fluctuation component for the single-mode optical fibers is attributable to the inherent polarization beat length determined by the difference of the phase velocities between the orthogonal HE11 and oHE11 modes. The power spectrum of the fluctuation is sharply peaked at the Fourier frequency determined by the polarization beat length of approximately 15 m, which is in good agreement with the analysis. The utilization of the power spectrum presents a new method to diagnose the polarization properties along the optical fiber.