A method is proposed for estimating and evaluating the service performance of ships at the initial design stage. The defects involved in the so called “seamargin” that has been used at the initial design stage of ships for many years are pointed out. To improve those defects, the authors introduce the probabilistic approach to estimate and evaluate the seamargin. The method proposed here is applied to evaluate the commonly used seamargin of an exsisting ore carrier engaged in the service between Japan and Australia.