  We present the first lattice-QCD calculation of the pion distribution
amplitude using the large- momentum effective field theory (LaMET) approach,
which allows us to extract lightcone parton observables from a Euclidean
lattice. The mass corrections needed to extract the pion distribution amplitude
from this approach are calculated to all orders in $m^2_{\pi} /P_z^2$. We also
implement the Wilson- line renormalization which is crucial to remove the power
divergences in this approach, and find that it reduces the oscillation at the
end points of the distribution amplitude. Our exploratory result at 310-MeV
pion mass favors a single-hump form broader than the asymptotic form of the
pion distribution amplitude.
