The unsteady flow over a cylinder confined by two parallel walls using direct numerical simulation (DNS) is investigated. The effects of two parameters, namely the Reynolds number and the blockage ratio, are the main focus for this study. The three-dimensional flow is simulated where the circular cylinder is placed symmetrically in a planar channel with a blockage ratio of β = 0.5 at Re = 200, 300 and 500. Quantitative analysis of the flow is conducted where the lift, CL and drag coefficient, CD and Strouhal number, St are investigated as β and Re are systematically varied. In the present study, it is found that different instability regimes are observed as the Re of the flow increases. The presence of a distinct shedding frequency is observed at Re = 200. However, increasing the Reynolds number further, the contribution of a range of frequencies surrounding the peak frequency appears as shown in the spectral analysis of the lift coefficient. The confinement effects played an important role in changing the properties of the wake. Counter rotating spanwise vortices are formed as a result of the interaction between the cylinder wake and the boundary layer of the walls in the channel which is entrained into the vortex street.