Off-axis　electron　holography　was　used　to　investigate　the　barrier　profile　of　the　Py/AlOx/ZrOy/Py　magnetic　tunnel　junctions　with　different　ZrOy　thicknesses.　The　tunneling　magnetoresistance　(TMR)　has　a　strong　dependence　on　bias　voltage　and　the　bias　voltage　for　maximum　TMR　is　shifted　from　zero.　This　shift　increases　with　ZrOy　barrier　thickness　due　to　the　increasing　barrier　asymmetry　in　the　junctions.　The　evolution　of　barrier　asymmetry　was　directly　observed　by　the　phase　change　of　the　off-axis　electron　holography,　which　unambiguously　shows　the　barrier　profile　changes　from　triangular　to　trapezoidal　shape　as　increasing　of　ZrOy　thickness.