  We point out that the recent Belle measurements of the $B_{d/s} \to J/\Psi
\eta^{(')}$ decays imply large pseudoscalar glueball contents in the
$\eta^{(\prime)}$ meson. These decays are studied in the perturbative QCD
(PQCD) approach, considering the $\eta$-$\eta'$-$G$ mixing, where $G$
represents the pseudoscalar glueball. It is shown that the PQCD predictions for
the $B_{d/s} \to J/\Psi \eta^{(')}$ branching ratios agree well with the data
for the mixing angle $\phi_G\approx 30^\circ$ between the flavor-singlet state
and the pure pseudoscalar glueball. Extending the formalism to the
$\eta$-$\eta'$-$G$-$\eta_c$ tetramixing, the abnormally large observed $B_d\to
K\eta'$ branching ratios are also explained. The proposed mixing formalism is
applicable to other heavy meson decays into $\eta^{(\prime)}$ mesons, and could
be tested by future LHCb and Super-$B$ factory data.
