When tidal turbines are utilized in the most energetic waters where there are significant waves, the assessment of the surface wave effects are of great concerns. The objective of this paper is to contribute to a fundamental understanding of surface wave effects on tidal turbines. A numerical model was developed based on the modified Blade Element Momentum theory with an inclusion of added mass effects, wave excitation forces and a one degree-of-freedom (DOF) simulation for turbine rotational motion. The experiments on a 1:25 scaled tidal turbine were performed in a towing tank. It is shown that the surface waves did not affect the average loads and power output, but caused severe periodical oscillations. The amplitudes of the cyclic thrust and torque could reach up to 50% of the mean value induced by the incident waves with period of 1.6 s and height of 14 cm. Non-dimensional response amplitude operators (RAOs) of thrust and torque were proved to be sensitive to submergence of the turbine. The wave induced torque and thrust tend to a fixed value when the incident wave length is much longer than the water depth, which provides an approximate assessment of the surface wave effects on tidal turbines.