Abstract: Three-dimensionality of flow past a line of obstacles is responsible for mixing in many practical systems, which in turn, for example, enhances the nutrient uptake and sediment transport within aquatic vegetation, increases the yield of turbine farms, and improves the heat transfer efficiency of exchangers. Despite these practical applications, the system was often modeled by two-dimensional (2-D) numerical simulations, which remove the three-dimensional (3-D) flow features. This paper presents, for the first time, results from three-dimensional direct numerical simulations that have been undertaken to examine the three-dimensionality of flow past a line of eleven cylinders (obstacles) for Reynolds number of 200 (Re_d = U∞d/ν, where U∞ is the velocity upstream of the cylinder, d is the cylinder diameter, and ν is the fluid viscosity). It is demonstrated that the flow past the line of cylinders displays very strong 3-D hydrodynamic features, which leads to large distinctions in instantaneous and time-mean flow and force characteristics from 2-D results. The critical transition from individual vortex shedding to two-row structure repeats along the line of cylinders in 2-D simulation but occurs only once in 3-D simulation. It is therefore recommended to use 3-D numerical simulations to investigate flow past a line of cylinders for Re_d ≳ 200.. Keywords: wake, fluid-structure interaction, three dimensionality