A pilot plant of a fluid energy mill having a horizontal-type of grinding chamber was studied and operated to obtain the design criteria for a scale-up of whole-sized plant. A mill having a 21.3 cm bore of the pulverizing zone was constructed. Milling operations for calcium carbonate, titanium oxide, and similar organic and inorganic materials were examined under various operating conditions. Fine product, as fine as 2-7 mum, could be consistently produced. A further study for the scale-up of a whole-sized plant (46 cm in diameter) is performed by using the test results obtained from the pilot plant facilities.