In rotor impact mills soft and semi-hard materials up to Mohs hardness of 3-4 can be ground without excessive wear at the grinding elements, The fineness of the grinding products depends to a great extent on the impact velocity. Other parameters such as the geometry of the grinding zone have to be considered as well, Especially for the ultra fine grinding an improved two stage rotor impact mill based on experimental investigations of Leschonski and Menzel has been developed, The design of the mill is characterized by radial channels for the acceleration of the material, a large number of small grinding elements and a minimized distance between rotor and stator. By a single passage through the present rotor stator system at a circumferential velocity of 94 m/s 75% of the limestone feed material (sieve fraction 1000-1500 microns) can be ground to a diameter smaller than 10 microns. The maximum particle size is 36 microns, The results indicate that the production of particles with maximum sizes below 10 microns should become possible by further improvements of the rotor stator geometry and, most important, by a significant increase of the rotor circumferential velocity. So a new mill allowing for circumferential velocities of up to 200 m/s has been designed, It will be investigated in the nearest future.