  We propose a method to determine the CKM matrix element $|V_{ub}|$ using the
global $q^2$ dependence of the dispersive bound on the form factors for $B\to
\pi l\nu$ decay. Since the lattice calculation of the $B\to \pi l\nu$ form
factor is limited to the large $q^2$ regime, only the experimental data in a
limited kinematic range can be used in a conventional method. In our new method
which exploits the statistical distributions of the dispersive bound proposed
by Lellouch, we can utilize the information of the global $q^2$ dependence for
all kinematic range. As a feasibility study we determine $|V_{ub}|$ by
combining the form factors from quenched lattice QCD, the dispersive bounds,
and the experimental data by CLEO. We show that the accuracy of $|V_{ub}|$ can
be improved by our method.
