In this study, comminution of coal with waterjets was performed to determine the factors that influence the particle size reduction. To carry out these experiments, a bituminous coal sample with an ash value of 31% was exposed to waterjet action within a conventional abrasive cutting head coupled with a specially designed cavitation chamber. Original coal feed of 371 mu m volume mean diameter and its subsamples of 59, 172, 323, and 527 mu m were used to investigate the effect of pressure on particle size reduction. For the purpose of this study, the comminution system was operated at 69, 138, 207, and 276 MPa. Physical properties such as particle size distribution and volume mean diameter were used for evaluation of the efficiency of comminution. Initial feed size was found as a dominant factor affecting the degree of size reduction. The increase in the cavitation system operating pressure resulted in finer products. Scanning electron microscopy studies of particle shape and morphology indicated transition from flaky to round and blocky particle shapes as a result of pressure increase. Severe clay dispersion was associated with all test conditions.