A Wells turbine blade for wave power conversion has hysteretic characteristics in a reciprocating flow. The hysteretic loop is opposite to the well-known dynamic stall of an airfoil. In this paper, the mechanism of the hysteretic behavior was elucidated by an unsteady 3-dimensional Navier-Stokes numerical simulation. It was found that the hysteretic behavior was associated with a streamwise vortical flow appearing near the blade suction surface. And also the effects of solidity, setting angle and blade thickness on the hysteretic characteristics of the Wells turbine have been discussed. (C) 2003 Elsevier Science Ltd. All rights reserved.