Abstract: A better understanding of the wake and power characteristics of wind turbines over hills could facilitate the layout design of wind farms in hilly terrain. High-fidelity large-eddy simulation (LES) is employed to investigate the effects of an upwind hill on the wakes and power performance of the turbine sited on the downwind-hill crest. In our pseudo-spectral LES framework, the wind turbine is parameterized by actuator disk model and numerical boundary condition of the terrain is modelled by immersed boundary method. It is found that our LES is in excellent agreement with the measured velocity and turbulence kinetic energy, outperforming the RANS method in terms of predictions of turbulence kinetic energy. The wake-center trajectory downwind of the hilltop is found to deflect downwards. Compared with the single-hill case, the wake recovery becomes faster in the double-hill cases, and its recovery rate first increases and then decreases as the distance between hills increases. It is seen that the Gaussian distribution is appropriate to describe the velocity deficit in turbine wakes in the hill cases, although a mismatch between them is found in the lower wakes due to the flow acceleration. Besides, the turbine rotor is found to be totally immersed in the upwind-hill wakes when the distance between hills reaches a certain distance. Furthermore, the power performance of the wind turbine is adversely affected by the upwind hill, which first drops and then rises as the distance between hills increases. The research improves our understanding of the wake characteristics as well as power performance of wind turbines over hills and is of guiding significance for the layout design of wind farms in hilly terrain.