  The branching fraction of the decay $\eta' \to \pi\rho$ is calculated in the
chiral quark NJL model. The decay exists due to the mass difference between
$u$- and $d$-quarks leading to $\pi^{0}-\eta^{'}$ mixing. Two different
approaches are applied: the approximate calculation of this process taking into
account the transition $\pi^{0}-\eta^{'}$ explicitly without dioganalisation
and a more precise approach consisting in the diagonalisation of the singlet
and octet states leading to the physical fields $\pi^{0}$, $\eta$ and
$\eta^{'}$. The obtained results are in satisfactory agreement with current
experimental data.
