The entrainment phenomena have been investigated across an interface between two-layered stratified flow induced by wind shear stress. The velocities of mean flow, turbulence and entrainment have been measured under three different conditions of water surface by using a wind-wave tank. When the entrainment velocity ue is expressed on the basis of the turbulent quantities at the interface, the turbulent entrainment coefficient is expressed, in an equation given in the paper, interms of effective byoyancy, turbulence intensity and integral lengthscale of turbulence at the interface. The result coincides with the relationship of entrainment due to oscillating grid turbulence, in which the mean flow does not exist. When, for practical purposes, the estimation of ue is made by using the mean velocity and the depth of mixed layer, another expression, also given in the paper, is derived.