  We calculate the branching ratios of $B_{s,d}\to \gamma \gamma$ decay in the
sequential fourth generation model. We find that the theoretical values of the
branching ratios, ${\rm BR}(B_{s}\to\gamma\gamma)$, including the contributions
of $t'$, are much different from the minimal standard model (SM) predictions.
The new physics contribution can provide one to two orders enhancement to the
standard model prediction of ${\rm BR}(B_{s}\to\gamma\gamma)$. But due to the
tiny values of 4th generation CKM matrix element $V_{t'd}^* V_{t'b}$, the new
physics effect on ${\rm BR}(B_{d}\to\gamma\gamma)$ is very small and can not be
distinct from the SM prediction. It is shown that the decay $B_s\to
\gamma\gamma$ can give the test of new physics signals from the 4th generation
model.
