  We study the production line shape of $B^\ast\bar B$ near threshold, where
the $B^\ast \bar B$ pair comes from the resonance $X_b$. Our study shows that
the line shape depends sensitively on the binding energy and the probability of
finding an elementary state in the physical bound state. Both of the two
parameters are crucial to identify the structure of $X_b$. Therefore, the line
shape measurement can shed light on the structure of $X_b$.
