  We measure the branching fractions of $B^{0} \to J/\psi \eta^{(}{}'{}^{)}$
decays with the complete Belle data sample of $772 \times 10^{6}$ $B\bar{B}$
events collected at the $\Upsilon(4S)$ resonance with the Belle detector at the
KEKB asymmetric-energy $e^+ e^-$ collider. The results for the branching
fractions are: ${\cal B}(B^{0} \to J/\psi \eta)=(12.3 \pm ^{1.8}_{1.7} \pm 0.7)
\times 10^{-6}$ and ${\cal B}(B^{0} \to J/\psi\eta') < 7.4 \times 10^{-6}$ at
90% confidence level. The $\eta-\eta'$ mixing angle is constrained to be less
than $ 42.2^{\circ}$ at 90% confidence level.
