We have studied the thermal stability of magnetic tunnel junctions with and without nano-oxide layer (NOL) using x-ray hpotoelectron spectroscopy (XPS). The con centration and chemical states of elements,in particular Mn,have been obtained b y angel-resolved XPS and peak decomposition technique.It is confirmed that Mn in the antiferromagnetic layer can diffuse into the pinned ferromagnetic layer and the insulating barrier layer when a magnetic tunnel junction without NOL is ann ealed at high temperature.However,the interdiffusion of Mn during the annealing process is suppressed by inserting a NOL between the antiferromagnetic and pinne d ferromagnetic layer,and then the thermal stability is improved.