An optical fiber of the present invention comprising a bare optical fiber and one or plural coating layers on the bare optical fiber, wherein Y, the Young's modulus of the outermost coating layer is in the range of 400 to 1000 and .mu., a coefficient of kinetic friction which occurs between the outermost coating layers is equal to or larger than 0.15, and further satisfying the following formulas (a) and (b); .mu..ltoreq.-0.9822 Log Y+3.45156 (a) .mu..gtoreq.-0.5418 Log Y+1.74128, (b) wherein the unit of Y is MPa. By the present invention, optical fibers with a low rate of occurrences of the attenuation discontinuity in a measurement result by OTDR and the method of measuring the coefficient of kinetic friction of optical fiber with high reliability are offered.