  In the 80's, Quillen constructed a de Rham relative cohomology class
associated to a smooth morphism between vector bundles, that we call the
relative Quillen Chern character. In the first part of this paper we prove the
multiplicativ property of the relative Quillen Chern character. Then we obtain
a Riemann-Roch formula between the relative Chern character of the Bott
morphism and the relative Thom form.
