Light Flavor Dependence of the Isgur-Wise Function

We present an investigation on the ligh flavor dependence of the Isgur-Wise function for $B_a\rightarrow D_a$ and $B_a\rightarrow D^{*}_a$ in the framework of QCD sum rules. It is found that the Isgur-Wise function for $B_s$ decay falls faster than that for $B_{u,d}$ decay , which is just contrary to the recent prediction of the heavy meson chiral perturbation theory. SU(3) symmetry breaking effects in the mass and the decay constant are also estimated.