The standard ocean wave spectrum is important for short-term prediction and long-term prediction of ships or floating structures in waves. The standard wave spectra used until today are basically based on P-M (Pierson-Moskowitz) type spectrum, that has ω^-5 shape in high frequency or equilibrium range. Recently, with the observed data, it has been suggested that the shape of spectrum is ω^-4 instead of ω^-5 shape at high frequency range. Technical committee of ITTC recommended to investigate the effect of this type of spectrum on ship responses. In this work, after introducing ω^-4 type spectrum, the resultant investigations are reported for the differences in short-term and long-term predictions of vertical bending moment and vertical acceleration of a typical ship.