On the R & D of a displacement-type monohull vessel suitable for high speed navigation, seakeeping qualities in rough seas are one of the most important subjects to be investigated. In high speed range up to 50 knots, there still remains uncertainty of the applicability of theoretical prediction methods such as Strip Method. Therefore, seakeeping model tests in head sea condition were carried out in regular and irregular waves for experimental studies. Frequency response functions and short-term responses in irregular waves were obtained about ship motion, acceleration, relative motion, and vertical wave loads. As the results of comparison between theoretical calculations and experimental results, it is revealed that the Strip Method can roughly estimate ship motions and wave loads even at F _n =0.6. For the prediction of short-term responses in irregular waves, however, experimental value of frequency response functions should be used. From the investigation of seaworthiness in irregular head waves of various sea states at various ship speeds, it is estimated that the displacement-type high speed ship has generally a good seakeeping quality on ship motions and vertical wave loads. However, vertical accelerations and relative motions at bow are estimated to be significant in high speed range in long-mean-period irregular waves.