  We study the correction to the radiative $\pi^0$ decay width due to finite
light quark mass. Using lattice QCD with the overlap fermion formulation, we
calculate the three-point function of the form $<PV_\mu V_\nu>$ in the
(Euclidean) momentum space, which corresponds to the $\pi^0\rightarrow
\gamma^*\gamma^*$ amplitude. To fit the lattice data, we use two different
modifications of vector meson dominance (VMD) ansatz. One is a combined form of
VMD with the next-to-leading order (NLO) chiral perturbation theory (ChPT), and
the other is a resummed form of pion-loop diagrams. We extract one of the low
energy constants in NLO ChPT, and estimate $\pi^0\rightarrow \gamma\gamma$
decay width including finite mass correction.
