The　barrier　potential　profiles　and　microstructure　of　Co　based　magnetic　tunnel　junctions　(MTJs)　annealed　up　to　340　degrees　C　have　been　studied　by　electron　holography　(EH)　and　high　resolution　electron　microscopy.　The　EH　results　reveal　that　the　annealing　process　can　well　improve　the　quality　of　interfaces　between　barrier　and　ferromagnetic　electrodes,　and　of　AlOx　barrier　itself,　which　are　responsible　for　the　improvement　of　tunneling　magnetoresistance　in　MTJs　after　anneal　at　280　degrees　C.