We have investigated the annealing effect on the magnetic tunnel junctions (MTJs) having different oxygen content. In All MTJs, enhanced tunneling magnetoresistance ratio could be obtained at the optimum temperature of each MTJ. From the results, it is closely related with the oxygen content in the oxide layer. The variation of an effective barrier width (S) was also critically depended on the oxidation state in MTJs. MTJ oxidized for 40s was not fully oxidized and showed the slight increase of effective S due to the reaction with an unreacted Al during annealing. However, the further oxidized MTJ showed the decrease of the effective S, and the extent of which was different with oxygen content in oxide layer.