A　non-associated　flow　rule　is　introduced　in　this　paper　to　characterize　the　yield　and　subsequent　plastic　deformation　of　porous　solids.　The　pressure　sensitive　yield　function　has　the　form　of　that　suggested　by　Gurson　and　Tvergaard.　A　macroscopic　plastic　potential　is　assumed　to　exist　for　porous　plastic　materials,　and　plastic　strain　increment　is　along　the　normal　to　the　macroscopic　plastic　potential.　The　yielding　function,　the　flow　rule　and　the　evaluation　of　the　void　volume　fraction　compose　the　non-associated　constitutive　model　for　porous　plastic　materials.　Detailed　finite　element　analysis　on　an　axisymmetric　unit　cell　is　performed;　the　unit　cell　is　a　representative　volume　element　of　porous　materials.　The　results　show　that　the　yield　and　deformation　of　the　cell　with　the　aspect　ratio　of　pi/4　is　isotropic,　and　the　yield　can　be　described　well　by　the　Gurson-Tvergaard　yield　function　with　the　parameters　q(1)　=　1.35,　and　q(2)　=　0.95,　The　post-yield　response　of　the　unit　cell　is　in　good　agreement　with　the　prediction　of　the　non-associated　constitutive　model.　(C)　1998　Elsevier　Science　Ltd.　All　rights　reserved.