Bispectra of wind-waves in wind tunnels were calculated in order to understand the characteristics of the nonlinear wave-wave interaction in actual wind-wave field. It is shown that the nonlinearity in wind-waves increases in magnitude with the development of wind-waves and that the characteristics of nonlinearity in wind-waves in the early stage of development differ from those in the late stage. It is shown that the bispectra are classified into five types (I∼V), and that the bispectral type changes from the type I to the type V as the wind-waves develop from the stage of the “initial-wavelets” to that of the “sea-waves”. The relations between frequencies of the component waves interacting each other are discussed in each bispectral type.