The effect of vibration and oscillation of an instrumented testing apparatus on horizontal axis tidal turbine rotor energy generation performance was examined and analyzed. Enforcement struts and an accelerometer were installed to measure and record vibration during the test with and without struts. Three rotors with a solidity (i.e., expanded area ratio, EAR) of 0.25, 0.4, and 0.51 were tested to obtain the rotor energy generation performance data. The analysis of the results is focused on the rotor open-water performance, in terms of the drag and power coefficient, respectively. The results show the reduced overall oscillation/vibration achieved an improvement in the measured power coefficient of about 6% and the drag measurement improved by about 3%. The allowable inflow velocity was also extended from 2 to 4 m/s. These results indicate that the enhanced vibration reduction method is effective and necessary for tidal turbine apparatus and its instrumentation.