Unsteady turbulent flow past 3D two circular cylinders in tandem arrangement is investigated numerically by using OpenFOAM LES model. Reynolds number is 3900 and center to center spacing between two cylinders varies from 2D to 3.5D and then to 5D. The flow characteristics have been investigated from upstream, downstream and gap between two cylinders. Pressure and velocity distribution, drag coefficient and lift coefficient at different spacing from both cylinders are compared with each other and experimental data. Results are in agreement with previous work. The concept of "Critical Spacing" is validated by this study at 3.5D spacing setup, which shows a sudden change flow pattern in the gap between two cylinders and eventually causes a drop on pressure profile. Subject study also confirms that 5D distance is sufficient for vortex formation from upstream cylinder and the interaction between two cylinders (impingement from upstream to downstream cylinder) is less obvious at such distance.