The estimation method of wave direction and angular spreading on the basis of covariances between the time series data of wave motions is numerically examined as the standard data processing technique of wave directional observation.
　In a bimodal wave system with wind waves and swell from different directions, the principal wave direction, one of the two representative directions defined by Longuet-Higgins, may indicate a direction outside the incident directions and unsuitable for the representative direction. Thus, the mean wave direction should be employed with the measurements of two horizontal and one vertical components of wave motions, when a surface buoy or a two-axis current meter is employed for wave directional obserbvaation. A new parameter called the mean spreading angle obtainbable from the covariances is proposed as the index of angular spreading.
　The applicability of the covariance method is verified with numerical simulations of wave profiles and motions with standard directional spectra. An estimate is made for the standard error of mean wave direction due to the statistical variability of sea waves. The statistical variation of cross spectral density if also clarified with numerical data.