The paper presents the results of coal milling tests in aspect of slurry fuel preparation. The tests were carried out with the use of vibrating mill with working chamber of 47.5 dm³. The influence of milling time, amount and composition of grinding aids on degree of fineness and particle size distribution has been investigated. It has been found that the efficiency of the grinding process (in this type of milling device) depends primarily on the milling time and the share of grinding aids and - to a lesser extent - on their polydispersity. The study allows to conclude that the grinding time, composition and share of used grinding aids enable to control the final grain size of coal which has an impact on apparent viscosity of coal water slurry.