Viscous flow past two circular cylinders of different diameters is simulated by using a finite element method. The diameter ratio between the small cylinder and the large one is 0.25. The Reynolds number based on the diameter of the cylinders is 500 for the large cylinder and 125 for the small cylinder. The gap between the small cylinder and the large cylinder ranges from 0.05 to 1.0 times the diameter of the large cylinder. The position angle of the small cylinder relative to the flow direction ranges from 0 to pi. The effects of the gap ratio between the two cylinders and the position angle of the small cylinder on drag and lift coefficients, pressure distributions around the cylinders, the vortex shedding frequencies from the two cylinders and flow characteristics are investigated. The magnitudes and frequencies of the fluctuating forces acting on the two cylinders are compared with those on a single cylinder of an equivalent diameter. (c) 2005 Elsevier Ltd. All rights reserved.