  The bounds on the form factors for $B \to \rho \ell \nu_\ell$ decay are
studied. Constrained by lattice data and a constrained conformal mapping, the
more informations can be obtained for $A_1(q^2)$ form-factor which dominates
the decay rate at large $q^2$. Specifically, we confirm a moderately increasing
behavior of this form factor.
