Monitoring and recording of ship operating conditions at all times on board can help estimate the seakeeping performance and its reduction. We discuss a stochastic model for the analysis and evaluation of monitoring data. The model is obtained by the analysis of wave height and ship responses measured on the actual ships, and expressed by the stochastic differential equation with three parameters. From a comparison of measured data, the validity of the model is indicated. Understanding parameters enables the evaluation and prediction of seakeeping performance. For examples, the long-term prediction and simulation of ship responses and the evaluation of ship operating condition are described.