  We present an optimal entanglement concentration ECP for an arbitrary
less-entangled W state. By two of the parties say Alice and Charlie performing
one parity check measurements, they can obtain the maximally entangled W state
with a certain probability. Otherwise, they can obtain another lesser-entangled
W state with another probability, which can be used to reconcentrated into a
maximally entangled W state. By iterating this ECP several times, it has the
maximal success probability. This ECP maybe an optimal one and is useful in
current quantum information processing.
