  We consider a sum rule for heavy-to-light form factors in soft-collinear
effective theory (SCET). Using the correlation function given by the
time-ordered product of a heavy-to-light current and its hermitian conjugate,
the heavy-to-light soft form factor zeta_P can be related to the leading-order
B meson shape function. Using the scaling behavior of the heavy-to-light form
factor in Lambda_QCD/m_b, we put a constraint on the behavior of the $B$ meson
shape function near the endpoint. We employ the sum rule to estimate the size
of zeta_P with the model for the shape function and find that it ranges from
0.01 to 0.07.
