Innovation of novel grinding systems like fluid energy grinding and fluidized bed grinding for the production of contamination-free, ultra-fine powder products necessitated an equally effective integral classification system for controlling the fineness of the products. Towards maintaining the high standards of purity of the product, it was found advisable to dispense with mechanical classifiers for such grinding systems. It is to facilitate this idea that a disk centrifuge-type pneumatic classifier was developed, with a classifier nozzle introduced at a specific location inside the classifier so that the suction created out of the expansion of air through this nozzle can be used for the on-line control of the product fineness and the classifier can be incorporated in any of the fluid energy-based grinding systems. The performance of the classifier was tested by mounting it on the grinding chamber of a circular fluid energy mill and by collecting the data for estimating the classification functions of particles, at different suction levels and material loading. A two-parameter model was used to relate the variation of the classification function with particle size and the coefficients of the model were characterized with the operating conditions.