The variation of the particle-wall friction factor along the riser is investigated in an Internal Circulating Fluidized Bed (ICFB) riser 1 m in length and 0.052 to in diameter. The results obtained are based on calculating the normal and the shear forces at the wall under dynamic conditions rather than the static ones usually obtained in shear box experiments. The strength of the method used resides in the measurement technique applied to measure the particle velocity field in the riser. The radioactive particle tracking program was developed for coaxial systems and is used to build dynamic pictures of particle trajectories in the vicinity of the wall of the ICFB riser. The experiments were conducted using sand (d(p)=250 mu m) and alumina (d(p)=170 mu m) materials in the gas velocity range between 2 and 12 m/s. The most common correlations for calculating the particle-wall friction factor are reviewed and compared to the results obtained in this work. The data obtained demonstrates that the particle-wall friction factor is not a constant value but changes along the riser and with change in the gas superficial velocity. The results also show the effect of the roughness of the wall surface and define the particle-wall friction factor area. (c) 2007 Elsevier B.V. All rights reserved.