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The flow field in a semi-circular duct is simulated by Large Eddy Simulation (LES) and its particle field is simulated by Lagrange particle tracking method. Reynolds number Reb (based on bulk velocity and hydraulic diameter) is 80,000 and Reτ (based on friction velocity and hydraulic diameter) is 3528. Particle diameter dp is chosen as 10, 50, 100, 500 μm corresponding to St as 0.10, 2.43, 9.72, 243.05. The results show that the intensity of the secondary flow near the ceiling is less than that near the floor because the ceiling is curved and able to inhibit the secondary flow. It is found that the difference between the semi-circular duct and the square duct is that the secondary flow in a corner of the semi-circular duct is not symmetrical along the diagonal although they have the same generation mechanism. Regarding the particles, small particles (dp ≤ 10 μm) are found to uniformly distribute in the duct, while large particles (dp ≥ 50 μm) preferentially distribute in the corner and floor center. The maximum particles (dp = 500 μm) fall on the floor quickly and their dispersion mainly depends on the secondary flow near the floor. Particle deposition in the corner depends on particle size due to the effect of secondary flow and gravity. The effect of lift force on particles becomes more significant for 50 and 100 μm particles in comparison with other smaller particles. In the end, the effect of secondary flow is found to be more significant to dominate particle behavior than that of flow fluctuation.