The at-sea experiment of the moored floating structure “POSEIDON” has been carried out at the Japan sea since 1986 for four years. This paper deals with the characteristics of directional wave spectra measured at the test field. Directional wave measurement in the field is important because of the increasing intrest in relation to maritime design procedures, and because of the growing number of model basins with multi-directional wave capabilities. In this experiment, three ultra-sonic type wave probes were installed as a line array at the sea bottom of test area, and the measured three time series of water surface elevations were used to analyze directional wave spectra by Maximum Likelihood Method (MLM). The directional wave spectra can be expressed as S (f) · G ( f , θ), where S (f) is frequency spectrum and G ( f , θ) is directional function. Using measured data, the characteristics of S (f) and G ( f , θ) were investigated intensively. The main results are as follows ; 1) The shape of the high frequency side of frequency spectrum S (f) is not proportional to f ^-5, contrary to expectations, but it is proportional to f ^-4. This was confirmed by applying Toba's 3/2 power law to measured data. The peak of frequency spectra become sharp when peak frequency shift to low frequency range. 2) Estimated Directional function G ( f , θ) can be approximated by the so-called Mitsuyasu Type function. The spreading parameter S becomes maximum near the peak frequency of S (f) , and it spreads arround 1025. The Wave direction estimatied from directional function, doesn't always accord with the wind direction by the refraction phenomena due to the shallow effect.