Large eddy simulation is used to study particle dispersion in fully-developed, square open duct flow at Reynolds numbers of 83 k,250 k based on the bulk velocity and duct width. Particle motion is governed by drag, lift, added mass force, pressure gradient force, buoyancy and gravity, with particle size in the range St = 0.032-2415.42. The results show that in the open duct cross-section, secondary flow dominates small particles' dispersion. For larger particles (St > 3.23), gravity plays a significant role in dominating their distribution and deposition rate, which increased with particle size. They tend to disperse better in the spanwise direction than vertical direction. Particles with St = 3.23-96.62 tend to accumulate in the corners, while those with St > 323 in Re = 83 k flow are prone to concentrate in the central regions of duct floor. It is observed that the Reynolds number could enhance particle dispersion in the whole cross-sectional plane. Near the bottom wall, particles with St = 12.93 are found to be also preferentially concentrated by near-wall turbulence, with particle size for this kind of accumulation increasing with Reynolds number. In addition, particles are found to be less dispersed in open duct flow in comparison with closed duct flow. Such trend increases with particle size and dimensionless time. A dynamic force analysis shows that the contribution of pressure drag and lift force gradually increases with particle size. Pressure drag can inhibit particle dispersion, whereas lift force has the opposite effect. The mechanism of particle dispersion in turbulent, open square duct flows is eventually elucidated. (C) 2019 Published by Elsevier B.V.