  B meson semi-leptonic rare decays are sensitive to new physics beyond
standard model. We study the $B^{-}\to \pi^{-}\mu^{+}\mu^{-}$ process and
investigate the Majorana neutrino contribution to its decay width. The
constraints on the Majorana neutrino mass and mixing parameter are obtained
from this decay channel with the latest LHCb data. Utilizing the best fit for
the parameters, we study the lepton number violating decay $B^{-}\to
\pi^{+}\mu^{-}\mu^{-}$, and find its branching ratio is about
$6.4\times10^{-10}$, which is consistent with the LHCb data reported recently.
