  We investigate what can be learnt about the $\eta$ -$\eta'$ mixing by means
of dispersive representation of axial anomaly. We show that our method leads to
the strong bounds for the $\eta-\eta'$ mixing angle:
  $\theta = -15.3^o \pm 1^o$. Moreover, our result manifests also a dramatic
dependence of the width $\Gamma_{\eta\to 2\gamma}$ on the mixing angle
$\theta$.
  This property explains how the relatively small mixing strongly effects the
decay width.
