The flow past two tandem circular cylinders of different diameters was simulated using the finite volume method.The diameter of the downstream main cylinder(D) was kept constant,and the diameter of the upstream control cylinder(d) varied from 0.1D to D.The studied Reynolds numbers based on the diameter of the downstream main cylinder were 100 and 150.The gap between the control cylinder and the main cylinder(G) ranged from 0.1D to 4D.It is concluded that the gap-to-diameter ratio(G/D) and the diameter ratio between the two cylinders(d/D) have important effects on the drag and lift coefficients,pressure distributions around the cylinders,vortex shedding frequencies from the two cylinders,and flow characteristics. The flow past two tandem circular cylinders of different diameters was simulated using the finite volume method. The diameter of the downstream main cylinder (D) was kept constant, and the diameter of the upstream control cylinder (d) varied from 0.1D to D. The studied Reynolds numbers based on the diameter of the downstream main cylinder were 100 and 150. The gap between the control cylinder and the main cylinder (G) ranged from 0.1D to 4D. It is concluded that the gap-to-diameter ratio G / D) and the diameter ratio between the two cylinders (d / D) have important effects on the drag and lift coefficients, pressure distributions around the cylinders, vortex shedding frequencies from the two cylinders, and flow characteristics.