$B_s (B_d) \to \gamma \nu \bar \nu $ }

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 $B_d$.