In order to reduce Greenhouse gas (GHG) emission from shipping sector, it is required to built and promote low GHG emission ships. On a project for 10 mode at sea in Japan, a calculation method on decrease of ship speed in actual seas, which is available in design stage, has been developed. For verification of the calculation method, full-scale onboard measurement has been conducted for a RoRo cargo ship and an oil tanker. As a result, it is confirmed that the calculation method is sufficiently reliable and can be accurately applied to both fine and blunt ships. Copyright © 2011 by the International Society of Offshore and Polar Engineers (ISOPE).