  The anomaly in the vacuum expectation value of the product of axial and two
vector currents (AVV) in QCD is investigated. The goal is to determine from its
value the $\pi^0 \to 2 \gamma$ decay width with high precision. The sum rule
for AVV formfactor is studied. The difference $f_{\pi^0} - f_{\pi^+}$ caused by
strong interaction is calculated and appears to be small. The $\pi^0 - \eta$
mixing is accounted. The $\pi^0 \to 2 \gamma$ decay width determined
theoretically from the axial anomaly is found to be $\Gamma(\pi^0 \to 2 \gamma)
= 7.93 eV$ with an error $\sim 1.5%$. The measurement of the $\pi^0 \to 2
\gamma$ decay width at the same level of accuracy would allow one to achieve a
high precision test of QCD.
