The spatial development of a 3D plane mixing layer in incompressible flow has been performed using a L.E.S. approach. The resolutions of the Navier-Stokes equations, written in velocity-vorticity formulation, is based on an original fractional step algorithm (Bertagnolio et al. (1996)). The results presented are obtained through the use of two subgrid scale models (a vorticity model and a mixed scale model). To validate the L.E.S. approach, a comparison with the experiments is provided. The main statistical values, expansion factor, Reynolds stress tensor and 3D arrangement are well predicted.