The influence of several grinding parameters on the product fineness in jet mill grinding was studied experimentally with a laboratory scale spiral type jet mill. The most significant variables in the jet mill grinding were feed rate, volumetric flow rate of grinding air and the height of an inside classification tube for grinding at constant pressure. Results showed that the height of the inner classification tube (vortex finder) is also a relevant variable in a spiral jet mill, and should be taken into account in optimising the process. The finest product size was obtained with smallest material feed rate, highest grinding air flow rate and with the shortest classification tube.