In this research, the effect of geometrical variation on the efficiency of an oscillating water column (OWC) wave energy converter was investigated by a water tank test. An inclined cylindrical OWC model was used for the test. The cylinder can be inclined from the horizontal level by 90°, 45°, and 18.4°. The top of the OWC model was sealed with an orifice plate in order to test the effect of power take off damping on the efficiency of OWC device. It is showed that the inclination of cylinder affected the OWC's resonance frequency more than that predicted by increased water plane area because of reduced gravity restoring force caused by inclination. Moreover, the volume of air chamber does not showed significant effect on the efficiency of OWC compared with the effect of inclination of OWC. Optimum nozzle area for the efficiency of OWC is increased proportional to square root of wave height with both inclined and not inclined conditions. Finally, supposed energy production is calculated using real sea state and inclination of water column has significant impact on the annual energy production.