Electron　holography　was　successfully　used　to　investigate　the　distribution　of　the　potential　of　the　barrier　in　magnetic　tunnelling　junctions　experimentally　and　theoretically.　We　got　a　clear　profile　of　the　potential　which　shows　a　＂well＂　in　reconstructed　phase　profile.　We　also　pointed　out　some　artefacts　in　electron　holographic　study　of　magnetic　tunnelling　junctions　and　suggested　some　possible　methods　to　resolve　them.