  We explore the impact of topflavour model to $B-\bar{B}$ mixings and rare
decays of $B$ mesons. By the flavour-changing neutral current interactions of
this model, the $B-\bar{B}$ mixing amplitudes are substantially affected while
it is hard to investigate the effects on rare decays. Violation of the
unitarity of the CKM matrix is also discussed. We find that the bound on
$|V_{td}|$ can be stronger in this model by combining the experimental bound of
the $B-\bar{B}$ mixing and the bound from violated unitarity.
