On-board measurement of fuel consumption of a ship has been carried out in a relatively severe sea condition. In the full scale experiment, the ship traveled on several courses to investigate the change of fuel consumption relative to the encounter wave angle. The result shows that the wave direction has a great influence on the main engine horse power and fuel consumption, and also shows a possibility of fuel efficiency prediction. In order to develop an eco-friendly navigation support system, results of Bayesian wave estimation are applied to fuel efficiency prediction. The Bayesian method does not require wave measurements but needs only ship motion data as input and the method is suitable for on-site wave estimation. Copyright © 2013 by ASME.