The high pressure roll mill, HPRM, a newly invented size reduction equipment, has been recently added to the list of comminution machines and is classified as a highly efficient equipment. Although the HPRM is now already being operated in several full-scale size reduction plants, it has not yet been fully characterized. This is because the performance of the equipment is affected by numerous parameters that have to be investigated and optimized. The present paper is concerned with the effect of one of the important parameters, that is the feed moisture, on the HPRM performance. A dolomite feed of –8 mesh size distribution was used at different moisture levels, up to 10%, to demonstrate the effect of feed moisture on the product characteristics, and the mill operating and design parameters such as the applied load, specific energy, roller speed, and rollers gap. It was found that the feed moisture has a definite effect on both the product characteristics and the mill operating and design variables at different set load levels. It was also found that the operating parameters of the mill are interrelated in a well defined combination, and differences in the size distribution parameters of the product are reflections of this interrelated combination.