  We study the loop induced rare decays $B_s(B_d)\to\gamma\nu\bar\nu$ within
the standard model. Both constituent quark model and pole model are used and
the branching ratios turn out to be of the orders of $10^{-8}$ for $B_s\to
\gamma \nu \bar \nu $ and $10^{-9}$ for $B_d \to \gamma \nu \bar \nu $. These
processes can be served to determine the decay constants of $B_s$ and $B_d$.
