After changing the heat treating atmosphere of nitrogen gas, argon gas and vacuum, the nitrogen contents, microstructural changes, hardness and corrosion resistance of 0.25wt.%N alloyed super duplex stainless steel have been investigated in the temperature range from 1050℃ to 1350℃. The nitrogen content showed to be increased up to 0.36wt.% after heat treating the specimen in nitrogen gas at 1200℃, while the decrement of nitrogen content in vacuum atmosphere was shown down to 0.03wt.% at 1350℃. After heat treating in the mixed gas atmosphere of argon and nitrogen at 1250℃, the surface γ phase existed as α+γ phase increased with increasing nitrogen gas content. The γ single phase appeared at the surface above 80%N₂gas, while the surface α single phase was shown below 20%N₂gas. When heat treating the specimen in nitrogen gas at 1050℃, the hardness of austenite phases increased above Hv 40 at the surface layer compared to the hardness of the core parts, while decrement of denitriding effect caused to the hardness nearly unchanged between surface and the core parts after heat treating in vacuum atmosphere. The surface γ single phase specimen showed superior corrosion resistance than the surface α single phase specimen. The surface α phase existed in the α+γ microstructure showed higher corrosion resistance after heat treating in the nitrogen gas atmosphere than the a phase heat treated in the argon gas and vacuum atmosphere.