This paper describes summarized results of the analysis on hull response data of a pure car carrier in service which is obtained through on board test of hull strength monitoring system. The system is currently being developed by Nippon Kaiji Kyokai and the purpose of installing this system on board of ships is to provide the navigator with accurate information on hull response in heavy weather so that the ship master can make an appropriate judgement for ship operation and maneuvers. Response data of deck stress, web frame stress and vertical acceleration at bow etc. is analyzed from view point of studying relation between hull response and deliberate loss of speed in rough seas. The results show that vertical acceleration at bow well correspond to deliberate loss of speed. It is also found that hull response is fairly influenced by wind direction and speed reduction is made mostly in head and bow wind. In full load condition speed reduction is made suddenly, on the other hand in ballast condition speed reduction is made gradually. It is anticipated that the hull strength monitoring system developed by the society will make fair contribution to safe and economical operation of ships.