  The advantage and feasibility of using $W$-boson production to extract unique
information on the flavor asymmetry of the $\bar u$ and $\bar d$ sea-quark
distributions in the proton are examined. The $W^+$ and $W^-$ production cross
section ratios in $p+p$ collisions are shown to be sensitive to the $\bar d/
\bar u$ ratios, and they are free from charge-symmetry-breaking and
nuclear-binding effects. The feasibility for measuring these ratios at the RHIC
and LHC proton-proton colliders, as well as the expected sensitivity to the
$\bar d/ \bar u$ ratios, are also presented.
