A fiber optic doppler anemometer comprises a source of coherent light, a directional coupler formed by the combination of a pair of single-mode optical fibers, and a photoelectric transducer. The first end of the first fiber receives an incident beam of light from the source and guides it through the directional coupler to the second end of the first fiber, which is located adjacent to a body of moving particles to be measured, where both the second end of the first fiber and the moving particles reflect a portion of the incident light back into the first fiber. The reflected light, which has a frequency different from that of the incident beam entering the first fiber, is directed back through the directional coupler which couples a portion of the reflected light toward the first end of the second fiber. The photoelectric transducer receives the light emerging at the first end of the second fiber and converts it into analogous electrical signals.This system may be used as a velocimeter using the Doppler effect to measure the velocity of moving particles and to measure the sizes of particles with Brownian motion.