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We discuss the effect of Fresnel reflection on BOTDR measurement. We show that the theoretical detection sensitivity with respect to Brillouin scattering is limited by the pulse modulation effect and phase noise of the light source that is incorporated into the Brillouin spectrum by way of the Fresnel reflection, when the Brillouin sensor is located near the Fresnel reflection point. We describe the detection sensitivity in the presence of Fresnel reflection in terms of the minimum detectable sensor length. In regard to the pulse modulation effect, we find that when the pulsewidth is short, the noise in the Brillouin spectrum is large and the detection sensitivity deteriorates. We derive quantitative equations and confirm the equations experimentally. When the pulsewidth is 10 ns with a 1-MHz linewidth laser, the boundary Fresnel reflection determining whether or not we can measure a 1-m sensor fiber near the Fresnel reflection is about 30 dB lower than that for a 100 ns pulsewidth. A 1-m sensor fiber near the Fresnel reflection could be measured with a 10-ns pulse only when the Fresnel reflection was very low at about . Moreover, the effect of the laser linewidth is also discussed analytically. The analysis suggests that a laser with a narrower linewidth will provide better sensitivity.