The particle size distribution of pulverized fuel in pneumatic conveying pipelines is an important physical characteristic closely related to mill energy economy, fuel flow property, combustion efficiency and pollutant emissions. In order to determine the size distribution of pneumatically conveyed fuel particles on an online continuous basis, an instrumentation system based on acoustic emission (AE) method is developed. This method extracts information about particle size distribution from the impulsive AE signals generated by the impacts of particles with a metallic waveguide introduced into the particle flow. Analytical modeling of the particle impact is performed in order to establish the relationship between the particle size and the peak AE voltage. With the particle velocity obtained from an electrostatic velocimetry system and the pulse magnitude determined using a peak detection algorithm, the particle size is computed directly using the developed model. Experimental results obtained with glass beads on a laboratory-scale particle flow test rig demonstrate that the system is capable of discriminating particles of different sizes from the AE signals. The system has several appealing features such as online continuous measurement, high sensitivity, simple sensor structure and low cost, which make it well suited for industrial applications.