  Rare B meson decays mediated by flavour changing neutral current (FCNC)
transition play interesting role to probe the flavour sector of the standard
model (SM). Generally at the tree level, FCNC processes are not allowed in the
SM but occurs at the loop levels. This gives an excellent hunting ground for
new physics (NP). From various experimental studies it is found that the FCNC
processes having quark level transition $b \to s$ are challenging. Here, we
investigate different kinematic observables like forward-backward asymmetry,
differential branching ratio and lepton polarization asymmetry for semileptonic
rare B decay modes $B_s \to \phi l^+ l^-$ and $B^+ \to K^+ l^+ l^-$ ($l=
\mu,\tau$) considering the contribution of Z-mediated FCNC. A noticeable
deviation of the observables for these decay channels from the SM value is
found because of non-universal $Z$-$bs$ coupling.
