The flow around two rectangular cylinders with aspect ratio of 0.5 in a tandem arrangement, was investigated using pressure measurements (in a wind tunnel) and flow visualizations (in a water tunnel) in the range of P/h from 0.6 to 4.0. Four flow patterns were identified, and processes of shear layers wrapping around, the shear layer reattachment, vortices wrapping around and vortices impingement, were observed. Mean and rms pressure distributions, flow visualizations and Strouhal numbers were presented and discussed. The paper revealed that the variations of Strouhal numbers were associated with the shear layers or vortex interference around two cylinders.