  We study the effect of the FCNC mediated $Z$ boson in the rare semileptonic
baryonic decays $\Lambda_b \to \Lambda l^+ l^-$. We consider the model where
the standard model fermion sector is extended by an extra vector like down
quark, as a consequence of which it allows CP violating Z mediated flavor
changing neutral current at the tree level. We find that due to this
non-universal $Zbs $ coupling, branching ratios of the rare semileptonic
$\Lambda_b$ decays are enhanced reasonably from their corresponding standard
model values and the zero point of the forward backward asymmetry for
$\Lambda_b \to \Lambda \mu^+ \mu^-$ is shifted to the left.
