Micro-feeding of fine powders has been studied experimentally by use of a vibrating capillary tube whose diameter is varied from 0.58 to 1.26 mm. Ultrasonic vibration of 20 kHz was generated by a piezoelectric transducer and applied to the tube so as to discharge micron or sub-micron particles. The mechanism of the powder flow in the vibrating capillary tube is deduced, such that a thin layer of particles near the inner wall acts as a lubricant through their micro-vibrations. Therefore, the inner powder easily passes through the tube by the force of gravity. The powder flow rate, velocity and packing fraction of particles in the capillary tube were also obtained. Furthermore, a new factor corresponding to the viscosity of fluid was introduced in characterizing the powder discharge. From the experiments using various fine powers, it was found that continuous operation of micro-feeding was possible even at a rate of milligrams per second.