To estimate the blade performance of a Darrieus-type cross-flow water turbine, differences between the characteristics of a Darrieus blade which moves along a circular path and that of a blade at rest in a uniform steady flow are examined quantitatively through the use of experimental data for three types of blades with different cambers or chord lengths. It is found that a unique stall character strongly affects the drag force acting on the Darrieus blade. A model of the changing pattern of the drag coefficient due to dynamic stall is proposed from the results of measurement for estimating the blade characteristics. As a result, the blade characteristics and performance of the Darrieus-type runner are predicted well by the use of aerofoil data in a uniform steady flow, taking into account the effect of a circling motion and the dynamic effect of the unsteady flow around the Darrieus blade.