Typical factors of ship structural failures are corrosion and fatigue clack of the members. Although these failures can be detected during an inspection and examination and repaired, it is very difficult to estimate when and where these failures break out in the ship structure. Evaluation of ship structural safety can be conducted by the assessment of the capability of the structure to take into account the effect of corrosion of members and the demand based on the encountered wave conditions. In this report authors study a structural safety assessment procedure of an aged ship based on structural monitoring data. This is the first report concerning the evaluation of wave-induced load of a 1000 ton class patrol ship equipped with structural monitoring system. The encountered wave conditions and sailing conditions are examined by the structural monitoring responses and log book records. Authors developed the estimation method for the relative angle between wave direction and ship heading course and the mean wave period of irregular waves from the correlation coefficient between pitching rate and midship deck stress measured by structural monitoring system. Based on the estimated sailing conditions and analyzed wave condition of long-term statistics of observed wave data recorded in the log books, the long-term wave-induced responses of the ship are estimated and compared with measured data.