Let k be a finite field of characteristic p > 0.We construct a theory of weights for overholonomic complexes of arithmetic D-modules with Frobenius structure on varieties over k.The notion of weight behave like Delignes one in the e-adic framework : first,the six operations preserve weights,and secondly,the intermediate extension of an immersion preserves pure complexes and weights.This is joint work with Tomoyuki Abe.