A smooth cubic hypersurface X of dimension >1 is unirational. The variety of lines F(X) on these hypersurfaces is an essential tool to understand the geometry of X. In dimension 3, the study of F(X) enables to prove that X is always irrational. In this talk we study the zeta function of F(X) and we obtain a simplified proof of the irrationality of a dense set of smooth cubic threefold. This is a joint work with D. Markouchevitch.