  In this paper, we discuss the partial separability and its criteria problems
of multipartite qubit mixed-states. First we strictly define what is the
partial separability of a multipartite qubit system. Next we give a reduction
way from N-partite qubit density matrixes to bipartite qubit density matrixes,
and prove a necessary condition that a N-partite qubit mixed-state to be
partially separable is its reduction to satisfy the PPT condition.
  PACC numbers: 03.67.Mn; 03.65.Ud; 03.67.Hk
