The primary objective of the present paper is to obtain an irregular wave train inducing the most likely nonlinear response at an arbitrary probability level in a short term sea state. The target response includes nonlinear combined load, e.g., von Mises stress, and cumulative elasto-plastic behavior of hull girder after the ultimate strength. To this end, FORM (First Order Reliability Method) is adopted following the works by Jensen, JJ. Then, the irregular wave train could be obtained as design points on limit state function by FORM. Once such an irregular wave train is obtained, the most likely response could be estimated easily either by simulation or experiment under the specified wave train. It is shown that such an irregular wave train could be specified for nonlinear response such as the combined load and the cumulative collapse behavior. It is observed that the irregular wave train which induces the most likely collapse behavior is characterized by one extreme peak.