Formation of fluidised bubbles in a fluidised bed has significant impact on the design and scale-up of the bubbling fluidised bed. To fully understand bubble motions in the fluidised bed, fluidised bubble dynamics in a gas-solid fluidised bed is investigated using CFD-DEM numerical simulation. To reveal the mechanism of bubble formation in the bubbling fluidised bed, the time autocorrelation approach is adopted to identify bubble occurrence at different bed height positions. A non-dimensional correlation accounting for the effect of local velocity field on the bubble size, which is obtained by modifying the Darton's model; is proposed for predicting the bubble size at given bed height positions. The non-dimensional correlation for predicting the size of the fluidised bubbles at various given bed height positions is obtained by regression based on the CFD-DEM modelling results. The simulation results indicate that small bubbles with high passing frequency dominate in the bottom region of the fluidised bed, while large bubbles are formed in the zone close to the free surface of the fluidised bed. The probability of the appearance of large bubbles increases as the height measured from the bottom of the fluidised bed increases, which may be attributed to the coalescence of small bubbles; this is also reasonably consistent with the results predicted using the Darton's model and experimental observations. It was observed that the sizes of the fluidised bubbles at the given height positions correlate with the local dynamic parameters characterised by the Reynolds number and the dimensionless height ratio. (C) 2017 Elsevier B.V. All rights reserved.