A comprehensive numerical simulation of the hydrodynamic performance of a caudal fin with unsymmetric flapping motion is carried out.The unsymmetrical motion is induced by adding a pitch bias or a heave bias.A numerical simulation program based on the unsteady panel method is developed to simulate the hydrodynamics of an unsymmetrical flapping caudal fin.A CFD code based on Navier-Stokes equations is used to analyze the flow field.Computational results of both the panel method and the CFD method indicate that the hydrodynamics are greatly affected by the pitch bias and the heave bias.The mean lateral force coefficient is not zero as in contrast with the symmetrical flapping motion.By increasing the pitch bias angle,the mean thrust force coefficient is reduced rapidly.By adding a heave bias,the hydrodynamic coefficients are separated as two parts:in one part,the amplitude is the heave amplitude plus the bias and in the other part,it is the heave amplitude minus the bias.Analysis of the flow field shows that the vortex distribution is not symmetrical,which generates the non-zero mean lateral force coefficient.