Abstract: Ramie (Boehmeria nivea) is an important source of plant protein in South China. Few studies investigating the physiological mechanisms of nitrogen utilization in feed ramie have been reported. To evaluate the activities of key enzymes associated with nitrogen metabolism in ramie, a potted experiment was conducted under different nitrogen levels. Two ramie varieties, H2000-03 which has high nitrogen utilization efficiency (NUE, 46.01%) and Ceheng Jiama with low NUE (15.81%) were treated with 0, 6, 9, 12 and 15 mmol/L nitrogen, and enzyme activity tested at the seedling stage (15 days), maximum growth (40 days) and at maturity (54 days). The results showed that as nitrogen concentration increased plant height, fresh yield and leaf protein content of both varieties increased and subsequently decreased or plateaued, indicating that the range of treatments imposed was able to meet the nitrogen requirement of the ramie plants. Plant height and fresh yield of H2000-03 were significantly higher than that of Ceheng Jiama under all treatments. The leaf protein content of H2000-03 was distinctly higher than that of Ceheng Jiama under a range of nitrogen levels (9-12 mmol/L). These results suggest that H2000-03 has better productivity under a range of nitrogen availabilities and more stable production under nitrogen deficient conditions. At different growth periods, the activity of NR (nitrate reductase) in H2000-03 and Ceheng Jiama gradually increased with increased nitrogen supply, while the activities of glutamine synthetase (GS), glutamate synthase (GOGAT), glutamic dehydrogenase (GLDH), catalase (CAT), superoxide dismutase (SOD) and peroxidase (POD) first increased and then decreased. The activities of POD, GS, GOGAT, CAT in H2000-03 under all nitrogen levels and the activities of SOD and POD under high nitrogen levels were higher than in Ceheng Jiama, while lower GLDH at all nitrogen levels and lower SOD and POD at low nitrogen levels were detected in H2000-03 compared to Ceheng Jiama. These results indicate that H2000-03 had a better physiological response and adaptability to different nitrogen fertility levels. The dynamic characteristics of ramie yield and quality were consistent with enzyme activity related to nitrogen metabolism and resistance which could be used to monitor and characterize nitrogen utilization differences among genotypes. Our research suggests that an appropriate nitrogen fertility range, rather than continuously increasing nitrogen should be ascertained to coordinate productivity with physiological characteristics. The activity of key enzymes during the rapid growth phase is suggested as an important trait for feed ramie breeding.