The mechanical properties of pellets produced under high pressure depend on many factors. The most important influencing factors are the type of wood, the particle size, the moisture content, the pressing pressure, the pressing speed, the deformation holding time, the punch diameter and the press temperature. Furthermore, the ratio of the channel diameter (D) and the channel length (L) has significant influence on the pellet density due to the relative wall friction which is necessary to ensure the counter force. In rolling presses the counter pressure is ensured by wall friction forces which depend on the channel length. To our best knowledge this problem has not been treated yet in details and, therefore, a theoretical and experimental investigation was undertaken to derive a generally valid relationship to describe the above phenomenon. The obtained relationship in dimensionless form is suitable to determine the necessary channel length to given channel diameter for different pellet densities or required maximum pressure using the chips of two wood species. The obtained results are in agreement with real channel diameter/length ratios used in practice.