The excessive acceleration is one of five stability failure modes for intact stability being discussed at IMO. The trial calculation for Direct Stability Assessment (DSA) and the model test have been carried out to verify two prediction methods of ship motions in the short-crested irregular waves for DSA. The ship motions in the short-crested irregular waves calculated by the linear superposition of frequency responses and the time domain simulation agree well with the results of the model test. Using experimentally validated prediction method and wave scatter table of North Atlantic, the long term probabilities have been calculated and the results obtained by two prediction methods become closer to each other. DSA evaluated by using the long-term failure probabilities obtained by two prediction methods are same.