Mixing layers are a classical case study in fluid mechanics. This kind of flow is characterized by stream wise vortex structures, called Kelvin Helmholtz vortices. In this paper we propose an analysis by the quadrant method of the second order quantities: velocity fluctuations of the mixing flow characteristics. An existing data-base of velocity field, obtained by PIV, is used. The data are filtered using a Gauss filter. The quadrant method was applied in order to find the mixing mechanism between the 2 layers.