This paper describes summarized results of the analysis on hull response data of a large bulk carrier in service which is obtained through on board test of hull strength monitoring system. The purpose on 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 at midship and vertical acceleration at bow etc. for 2 years is analyzed from view point of studying relation between hull response and loss of speed in rough seas. The results show that extreme value of vertical acceleration at bow seems to have bounds in rough seas due to loss of speed. It is also found that speed loss ratio of the ship in rough seas is fairly large compared with container carrier and pure car carrier. Theoretical calculation by Strip Method is made in addition, and the results show considerable decrease of vertical acceleration at bow due to speed reduction in head and bow waves.