Motion of glass beads of 0.4 mm mean diameter is observed in the central region of a circulating fluidized-bed (CFB) riser by coupling a reflective-type particle image scope (RPIS) with a highspeed video system of 3000 frames s(-1) recording rate. Transient changes in the particle velocity are measured under both dilute and dense conditions. The observed speed of ascending particles ranges from 0 to 3.0 m s(-1) at a superficial gas velocity of 6.0 m s(-1). Descending particles are also observed, and their speed ranges from 0 to 1.6 m s(-1) The structure and motion of particle swarms are analyzed with a special emphasis on particle collision. It is confirmed that the speed, moving direction, and packed density of the particle swarms are subjected to persistent time variations. Various patterns of particle motions are identified in the CFB riser, induced by a diversity of collision mechanisms. These include momentum transfer from a colliding particle to other particles being contacted, grazing followed by particle rotation, roundabout motion of a particle around another, simultaneous collision involving more than three particles, and successive particle collisions.