Particle rotation plays an important role on several aspects in gas-solid two-phase flow. However, it has not been paid much attention due to a lack of appropriate measurement methods. An attempt has been made in the present paper on the experimental study of particle rotation characteristics in a cold pilot-scale Circulating Fluidized Bed (CFB) riser, by using a high-speed digital imaging measurement system. It is found that one can measure rotation speeds manually for particles with special speckles on their surfaces or irregular shapes by observing particle image sequences. A dual-frequency imaging method was presented to enlarge the maximal measurable rotation speed at finite frame frequency and the measured rotation speeds are validated theoretically. Furthermore, particle rotation characteristics in a cross-section in upper dilute-phase zone were analyzed statistically. The results show that the average particle rotation speed is about 300 rev/s with the top speed of 2000 rev/s, when the superficial gas velocity U-g, external solids mass flux G(s) and average particle diameter are 5 m/s, 1.5 kg/(m(2) s) and 0.5 mm, separately. The average particle rotation speed near the wall area is higher than that in the center area at the testing cross-section. Those particles, with either smaller size or higher radial component of translational speed, may have higher average rotation speed. The average rotation speed of irregular particles is apparently higher than that of the spherical ones. (c) 2007 Elsevier B.V. All rights reserved.