Viscous flow around an oscillating vertical cylinder with a disk attached at keel is studied in detail by direct numerical simulation based on finite difference method. Over the range of flow parameters investigated, viz. Keulegan-Carpenter number (KC) from 7.5 x 10(-4) to 0.75 and beta = 1.585 x 10(5), three shedding modes were observed. These modes, named independent, interactive and uni-directional, are found to be governed by both flow parameters (KC, beta) and the geometry of the disk, such as aspect ratio, t(d)/D-d. The mechanism for the initial condition dependence of the asymmetric uni-directional vortex shedding is analyzed. The effects of KC number and aspect ratio of the disk on the vortex shedding angle are also presented. (C) 2003 Elsevier Ltd. All rights reserved.