Abstract: The effect of velocity spread on the main parameters of gyrotron oscillators is descri-bed briefly. Measures to reduce the velocity spread are discussed. A new method of measu-ring velocity spread of electrons in a cycloiding electron beam is given. By use of a high precision radial-gauge with multi-section collector, the beam transverse velocity distribution can be accurately measured. This method is characterized by the fact that the space charge effect wil not interfere with the experimental results, so the beam transversevelocity spread in a gyrotron under actual operating condition can be determined. Thus the method is superior to the retarding-field method and pin-hole-camera method. A special equipment is designed for this measurement.