The principal reason for the backscattering in an optical fiber is the Rayleigh scattering which is caused by non-uniform density of structure material of an optical fiber and diameter variations of the optical fiber-core along an optical fiber axis. The backscattering signal which is detected by the optical time domain reflectometer system(O.T.D.R) conatains information about both tha actual decay of power and the diameter variation along the optical fiber. In this paper, the O.T.D.R. system with 2x2 fiber directional coupler, timing control unit and gated integrator is used to measure diameter variations of an optical fiber.