  We propose a scheme to directly measure the entangled W states with
cross-Kerr nonlinearity mediums. This scheme can measure three-photon,
four-photon, and even N-photon W states in one step. Only PBSs and cross-Kerr
nonlinearity mediums are used in this scheme, which is feasible for
experiments. We describe the three-photon W states measurement process and
extend it to four-photon and N-photon W states.
