SUMMARYThis paper is concerned with the problem of shape optimization of two-dimensional flows governed by the time-dependent Navier-Stokes equations. We derive the structures of shape gradients for time-dependent cost functionals by using the state derivative and its associated adjoint state. Finally, we apply a gradienttype algorithm to our problem, and numerical examples show that our theory is useful for practical purposes and the proposed algorithm is feasible in low Reynolds number flows.