A plug flow regime may occur when a pneumatic conveying system operates with low gas velocities and high pressure gradients. Although noted for its high efficiency, there is insufficient theoretical knowledge to fully exploit this advantage in the design process. The present work introduces a fundamental theoretical and experimental analysis on the friction force of a vertical plug flow. In order to investigate the friction force of individual plugs a special experimental apparatus was designed and constructed that enables us to conduct experiments with plug lengths up to 1 m either with or without air flow through the plug. The data on the air velocity flowing through the plug, pressure gradient over the plug and the force required to move the individual plug were collected and processed on line. The experimental results show that the plug friction force is significantly dependent on the pipe diameter, wall friction coefficient, stress ratio and plug length. The theoretical results of the plug friction force which were calculated by using a general equation of the active and passive stress ratios show significant under and over-predicted results, respectively. A reverse engineering method was used to characterize the true stress ratio values. It was found that the stress ratio for coarse materials can be presented by a power law relationship of the ratio between the plug length and the pipe diameter. (C) 2011 Elsevier B.V. All rights reserved.