  We first investigate the semileptonic annihilation decay B_c \to \eta' l\nu
in QCD. We find that the $\eta'$ momentum distribution is peaked within its
small recoil region due to the loop effects. The branching ratio is estimated
to be $ Br(B_c \to \eta' l\nu)=1.6\times 10^{-4}$ for $l=\mu,e$, which is
accessible at CERN LHC.
