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Nitridation and oxidization processes of metal Al film to form very thin insulating layer in magnetic tunnel junctions (MTJs) are investigated, using microwave-excited plasma. The plasma nitridation process provides wider controllability than the plasma oxidization for the formation of ultrathin insulating layer in MTJs, because of the slow nitriding rate of metal Al films, comparing with the oxidizing rate of them. As a result, high tunnel magnetoresistance (TMR) ratios of 49% and 44% with respective resistance-area product (R×A) of 3×104 Ωμm2 and 6×103 Ωμm2 are obtained in the Co-Fe/Al-N/Co-Fe MTJs. These values are comparable to those of MTJs with Al-O barriers. The remarkable feature of the nitridation process is its larger amount of plasma exposure, needed to obtain similar resistance in MTJs, comparing with the oxidization process. It means wider controllability of the plasma nitridation for the formation process of very thin tunnel barriers. We conclude that Al-N is a hopeful barrier material to realize MTJs with high TMR ratio and low R×A for high-performance MRAM cells.