A very accurate mathematical model for the evaluation of the backscattering by a perfectly conducting 90°dihedral corner has been developed. It has been obtained by adding the Physical Theory of Diffraction (PTD) correction term to the Improved Physical Optics (IPO) model, which takes into account also the lighting of each face by the rays diffracted from the edge of the other one. The agreement of such a model with the experimental results has been found very good.