  We present the first direct lattice-QCD calculation of the Bjorken-$x$
dependence of the valence quark distribution of the pion. Using large-momentum
effective theory (LaMET), we calculate the boosted pion state with long Wilson
link operators. After implementing the one-loop matching and meson mass
corrections, our result at $m_\pi \approx 310$ MeV is in agreement with those
extracted from experimental data as well as from Dyson-Schwinger equation in
small $x$ region, but a sizeable discrepancy in the large $x$ region. This
discrepancy provides a nice opportunity to systematically study and disentangle
the artifacts in the LaMET approach, which will eventually help to discern
various existing analyses in the literature.
