One of the dangerous situations for ships navigating in following and quartering seas is to encounter occasionally a group of high waves, i.e. high run, which may be larger than those observed in ocean wave itself. The spectral analyses in frequency domain and the numerical simulation in time domain of encounter waves have been carried out, to investigate the statistical properties of the encounter wave grouping phenomena. According to Longuet-Higgins' procedure on ocean waves, the band width parameters of the encounter wave spectra have been used to calculate the probability of occurrence of group length and high run as the functions of various operation conditions of ship, such as ship speed/ wave period ratio, V/T, and encounter angle. The results have been confirmed by the statistical analyses of simulated time series of encounter waves that the probability of occurrence of high runs in following sea becomes higher in the range of V/T between 1.0 and 2.0 with the highest value at V/T equal 1.45. The influences of directional distribution of wave energy, i.e. the effects of short-crestedness of ocean waves have been also investigated and it has been shown that the increment of encounter wave groupiness is less, as the directionality of waves becomes wider.