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In the present paper, two- and three-tlimensional numerical simulations of the flow interference between four cylinders in an in-line square arrangement at Re = 200 are performed.Assisted with the two-dimensional (2-D) numerical simulation, the mean and fluctuating forces, Strouhal number (S,) and vortex shedding pattern in the wake for each cylinder were analyzed with the spacing ratio (L/D) ranging from 1.5 to 6.O.It was found that, four different vortex modes ( viz., flip-flopping, shielding anti-phase-synchronized, in-phasesynchronized and anti-phase-synchronized) gradually appear with the increase of the L/D ratio.The average drag coefficient of the upstream cylinders is larger than that of the downstream cylinders, while the downstream cylinders usually undergo serious fluctuating forces.When the L/D ratio ranges from 3.0 to 4.0, the dominant frequency of the drag coefficient is equal to the value of St of upstream cylinders.This indicates that a simuhaneous resonance in the in-flow and cross-flow directions may occur for some single structures of a multi-body oscillating system.For the 3-D numerical simulation, the L/D and aspect ratios are kept constant as 5.0 and 10,respectively.It was found that some vortices are formed in the wake of the upstream cylinders.Besides, with the same spacing ratio, the calculated drag coefficient and lift coefficient fluctuation are slightly larger than the 2-D results, but with a phase difference.