Monoplane and biplane Wells turbines for wave power conversion have hysteretic characteristics in a reciprocating flow. In this paper, the mechanisms of the hysteretic behaviors were elucidated based on unsteady 3 dimensional Navier-Stokes numerical simulations. For the monoplane Wells turbine, the hysteretic loop is opposite to the well known dynamic stall of an airfoil. It was found that the hysteretic behavior was associated with a streamwise vortical flow appearing near the suction surface. For the biplane Wells turbine, the hysteretic behavior was similar to that of the monoplane at lower attack angles, but the hysteretic loop similar to the dynamic stall was observed at higher attack angles, which was attributed to unsteady flow separation near the hub and the trailing edge of the suction surface of the upstream blade. (C) 2003 Elsevier Ltd. All rights reserved.