In most atomization processes the instability of a liquid sheet or, in case of some air-blast atomisers, of an interface is involved. Prediction of the growth of these instabilities is a main step in the physical modeling of a spray system. Such calculations are mainly achieved by use of linear theories. This has been proved to be efficient in order to get instability "scales" (wavelength) providing "mean" drop sizes (Dumouchel et al). If further information is required, a non linear approach is needed. A non linear approach is proposed dealing with the case of an initially planar liquid gas interface (Kelvin-Helmholtz problem). A numerical method, based on discretized vortex sheet analysis has been employed, as previously developed by several authors. The non linear growth of a single mode is shown, the evolution from the initial sinusoidal shape to a ligament-like form is well predicted. Calculations have been achieved down to smaller gas liquid density ratios than previous work. The influence of the initial amplitude of the perturbations on the final shape of the interface is examined. It is shown that a sole criterion for validity of a linear assumption, which would be based on initial amplitude of a single mode is not sufficient: the shape is also to be considered. The non linear simultaneous growth of several different modes (initially sinusoidal) in an integer ratio has been studied. Observed results are interpreted in connexion with a spatial FFT of the interface shape.