This paper deals with a new definition of the Radar Cross Section suitable for surface wave propagation in the HF band. Indeed, it can be shown that the classical definition of the RCS cannot converge for this kind of propagation. This is an issue for the performance assessment of High Frequency Surface Wave Radars. Thanks to the analysis of different wave propagation models, the differences between the space wave propagation and surface wave propagation have been highlighted. The required modifications of the RCS should then be performed. The new definition is explained in the paper and it is shown that it is suitable for the surface wave propagation. Moreover, this new definition has been used for the computation of the RCS of 3D naval targets and the obtained results will be presented during the conference.