Solids motion and flow structures in fluidised beds are of significant importance to combustion, pyrolysis or gasification of solid fuels, especially considering that low density, coarse biomass particles need to be well-mixed in the bulk bed of mostly sand. In this study, the investigation focused on the effect of air distributors on solid/gas motion, flow structure and bubble behaviour in a cylindrical fluidised bed by using the non-intrusive positron emission particle tracking (PEPT) technique. The results indicate that the flow structure and bubble behaviour vary with the hole size of air distributors. When glass beads with a size of 250-500 micron were used, three types of flow structures have been observed. Pattern A was observed when the hole size of air distributors was greater than 200µm. A single large circulation cell was observed within the whole bed. The particles moved upwards at one side of the bed to the splash zone, and returned to the bed bottom along another side of the bed. Pattern B was observed when the hole of the air distributors was in the range of 15-200µm. The particles moved upwards across the whole area immediately above the air distributor at a relative uniform velocity, and then travelled upwards in the centre of the bed along the axis to the splash zone. Pattern C was observed when the hole size of air distributors was less than 15 micron. Solids circulated at the bottom and the top sections of the bed, respectively. The fluidised bed can be seen as three different regions. A jet of gas entered the bed bottom through the air distributor with a size less than 15 micron expands along the wall rather than straight upwards through the centre. The air drove the solids to a bed level of 50-60 mm along the annulus. At the intermediate height of 60-100mm, the upward solid flow encountered the downward flow from the top section of the bed at the annulus. The two solid flows merged and changed the direction towards the bed centre. The solids in this region were mixed and then divided into two portions. One of them was involved in the circulations at the bed bottom as a return flow, while the other was involved in the circulations at the top region. The heat and mass exchange were achieved by three different mechanisms. Within a circulation cell, the exchange can be achieved due to the internal circulations. In the intermediate height of the bed, the encounter of upflow and downflow at the annulus caused a chaotic motion of solids with no uniform flow direction, and the solids from the top and bottom sections met, and then randomly diverged into the top or bottom sections. By this mechanism, particles from the bottom of the bed were mixed with those from the top, and then redistributed to the four vortices. The bubbles or air streams ascending from the bed bottom were split and forced to shift to the central of the bed by the solid flow returning from top region. The split gas gave opportunity for gas-solid contact. Thirdly, solids also exchanged between the neighbouring vortexes.