Long period waves with period of 1 to about 5min sometimes cause the termination of cargo loading and unloading. The mechanism of such damage is induced by the resonance of the long period waves and the mooring system composed of the large cargo vessel and mooring ropes. The mooring ropes hae been broken when the horisontal motions of vessels become large by the resonances. Several countermeasures are proposed to prevent the danage by long period waves. The extension of breakwaters, the development of new material for mooring lines, the construction of wave abdorbing beach in harbor et al. are capable to be considered as the contermeasures. Meanwhile, the characteristics, especially the spectral type for long period waves become important to establish the appropriate countermeasures. In the paper, the standared energy density level of long period waves are proposed using the field data obtained in several ports. The proposed energy level is uniform in the frequency range smaller than the boundary frequency fb.
　The proposed standard long period wave frequency level is applicable to estimate the harbor tranquility for long period waves. The estimaed probability of exceedance for long period waves becomes identical to that obtained from field observation. Therefore, the proposed long period frequency level is applcapable to estimate the harbor tranquility level for long period waves.