Abstract: The test ultra-low carbon austenite stainless steel 316L (/%: 0. 010 〜0. 013C, 17. 50 〜17. 67Cr, 10. 10-10. 60Ni, 1.89 〜2. 02Mo, 0. 020 - 0. 201 N) is melted by a 200 kg vacuum induction furnace and hot rolled to 15 mm steel plate by 550 mm mill. The structure and impact toughness of steel 316L heat treated at 1 050 T for 40 min, water cooling, then at 750℃ for 25 ~ 100 h, air cooling has been studied. Results show that with increasing ageing time the impact energy of steel 316L decreases ； with same ageing time the impact energy of 0. 201% N high nitrogen steel 316L is lower than that of 0. 020% N low nitrogen steel 316L, with ageing at 750 ℃ for 100 h, the impact energies of high and low nitrogen steel 316L are respectively 40 J and 150 J ； the main precipitates in low nitrogen steel 316L are 3 ~5 μm carbides dispersed distributed at grain boundary and in grain and the main precipitates in high nitrogen steel 316L are 100 〜200 μm carbo-nitride and σ- phase distributed along grain boundary.