  We test the convergence property of the chiral perturbation theory (ChPT)
using a lattice QCD calculation of pion mass and decay constant with two
dynamical quark flavors. The lattice calculation is performed using the overlap
fermion formulation, which realizes exact chiral symmetry at finite lattice
spacing. By comparing various expansion prescriptions, we find that the chiral
expansion is well saturated at the next-to-leading order (NLO) for pions
lighter than $\sim$450 MeV. Better convergence behavior is found in particular
for a resummed expansion parameter $\xi$, with which the lattice data in the
pion mass region 290$\sim$750 MeV can be fitted well with the
next-to-next-to-leading order (NNLO) formulae. We obtain the results in
two-flavor QCD for the low energy constants $\bar{l}_3$ and $\bar{l}_4$ as well
as the pion decay constant, the chiral condensate, and the average up and down
quark mass.
