A　novel　6　degree　of　freedom　(6-DOF)　passive　vibration　isolator　is　studied　theoretically　and　validated　with　experiments.　Based　on　the　Stewart　platform　configuration,　the　6-DOF　isolator　is　constructed　by　6　X-shape　structures　as　legs,　which　can　realize　very　good　and　tunable　vibration　isolation　performance　in　all　6　directions　with　a　passive　manner.　The　mechanic　model　is　established　for　static　analysis　of　the　working　range,　static　stiffness　and　loading　capacity.　Thereafter,　the　equation　of　motion　of　the　isolator　is　derived　with　the　Hamilton　principle.　The　equivalent　stiffness　and　the　displacement　transmissibility　in　the　six　decoupled　DOFs　direction　are　then　discussed　with　experimental　results　for　validation.　The　results　reveal　that　(a)　by　designing　the　structure　parameters,　the　system　can　possess　flexible　stiffness　such　as　negative,　quasi-zero　and　positive　stiffness,　(b)　due　to　the　combination　of　the　Stewart　platform　and　the　X-shape　structure,　the　system　can　have　very　good　vibration　isolation　performance　in　all　the　6　directions　and　in　a　passive　manner,　and　(c)　compared　with　the　simplified　linear-stiffness　legs,　the　nonlinearity　of　the　X-shape　structures　enhance　the　passive　isolator　to　have　much　better　vibration　isolation　performance.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.