Microfeeding can be used, inter alia, for solid freeforming, colour management and pharmaceutical dosing. In this work, a computer-controlled microfeeding system using ultrasonic vibration of a capillary was built. This paper describes the powder structures in the microfeeding process and defines the main processing factors affecting the mean dose mass. The experimental results show that the particle structures in the capillary tube can be divided into three types: arching, plugging and blocking. The nozzle diameter, transmission fluid depth, waveforms, voltage amplitude, frequency and oscillation duration all influence the dose mass. Among these factors, the nozzle diameter, voltage amplitude and oscillation duration can be used to control the dose mass.