We study the form factors of heavy-to-heavy and heavy-to-light weak decays using the light-front relativistic quark model. For the heavy-to-heavy B~D ' semileptonic decays we calculate the corresponding Isgur-Wise function for the whole kinematic region. For the heavy-to-light B -+ P and B~V semileptonic decays we calculate the form factors at q = 0; in particular, we have derived the dependence of the form factors on the 6-quark mass in the m&~oo limit. This dependence cannot be produced by extrapolating the scaling behavior of the form factors at q "using the single-pole assumption. This shows that the q dependence of the form factors in regions far away from the zero recoil could be much more complicated than that predicted by the single-pole assumption.

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Efforts to predict the rare exclusive decay B -t K'y from the well-known inclusive decay b + sy are frustrated by the effect of the large recoil momentum. We show how to reduce the large uncertainty in calculating this decay by relating B -+ K*y to the semileptonic process B + pep using the heavy-quark symmetry in B decays and SU(3) flavor symmetry. A direct measurement of the q 2 spectrum for the semileptonic decay can provide accurate information for the exclusive rare decay. PACS number(s): 13.40. Hq, 11.30.Hv, 13.20.Jf The inclusive rare decay B + X , y is now well understood in the context of the standard model [ l ] and the experimental upper bound [2] of 8.4 x 10W4 for the branching ratio is already playing a n important role [3] in constraining the parameters of models other than the standard model. On the other hand, the most likelv experimental observation t o be made will be the exclusive decay B + K * y . The recent limit from the CLEO Collaboration [4] of the branching ratio for this mode is 0.92 x It is this exclusive rare decay B -+ K ' y , however, which is the least well known theoretically due

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