Abstract: A new hybrid finite difference and vortex method,which is based on domain decom-position and proposed by the authors,is used for calculating the oscillating flow around a circular cylinder at various Keulegan-Carpenter numbers (Kc=2~24) respectively.The rules of variation of vortex motion patterns with Kc numbers are studied systematically.The vortex motion patterns in asymmetric regime,single pair (ortransverse) regime,double pair (or diagonal) regime and three pairs regime are first successfully simulated in China.The calculated drag and inertial coefficients are in better agreement with experiments than other recent numerical results.