In order to further reveal the solid flow behaviour in COREX, the phenomenon of small particle percolation under gravity, which involves in COREX process, is investigated by means of discrete element method. The effects of material properties on the percolation behaviour are simulated in this paper, including diameter ratios of percolation to packing particles, damping and sliding friction coefficients between percolating and packing particles. The trajectories of percolation particles under different simulation parameters are also examined. The results show that the mean percolation velocity of small particle moving down through the bed is a constant. It is shown that increasing the diameter ratio would decrease the mean percolation velocity and increase the radial dispersion and the mean residence time. With the increase of damping coefficient, the mean percolation velocity increases and the residence time and radial dispersion of the percolation particles decrease. The sliding friction coefficient has limited effects on the percolation behaviour. From the analyses of the trajectory of small particle, it can be seen that small particle would move down slowly and even trend to be static as the diameter ratios of percolation to packing particles increase. With the decrease of damping coefficient, the trajectory of small particle would deviate from the centre and close to the boundary of the bed.