  We study $B\to J/\Psi \eta^{(\prime)}$ decays and suggest two methods to
determine the $\eta-\eta^{\prime}$ mixing angle. We calculate not only the
factorizable contribution in QCD facorization scheme but also the
nonfactorizable hard spectator corrections in pQCD approach. We get the
branching ratio of $B\to J/\Psi \eta$ which is consistent with recent
experimental data and predict the branching ratio of $B\to J/\Psi
\eta^{\prime}$ to be $7.59\times 10^{-6}$. Two methods for determining
$\eta-\eta^{\prime}$ mixing angle are suggested in this paper. For the first
method, we get the $\eta-\eta^{\prime}$ mixing angle to be about
$-13.1^{\circ}$, which is in consistency with others in the literature. The
second method depends on less parameters so can be used to determine the
$\eta-\eta^{\prime}$ mixing angle with better accuracy but needs, as an input,
the branching ratio for $B\to J/\Psi \eta^{\prime}$which should be measured in
the near future.
