Although excessive use of N fertilizer can result in the rapid decrease of N use efficiency and other serious environment problems, N fertilizer is yet the most important and largest input in modern rice production. Previous researches have been demonstrated that variation in N use efficiency existed among rice genotypes. Therefore, it is of urgent importance to elucidate the physiological mechanism of rice N absorption and utilization for increasing N use efficiency. In this research, field experiment with 225 kg ha^-1 N fertilizer application was carried out in 2006 on the farm of Yangzhou University, Jiangsu province, China. Twelve rice genotypes (six N efficient and six N inefficient) selected from one hundred and twenty rice cultivars grown in Yangzhou during 2004 and 2005 were adopted to investigate the characteristics of N absorption and accumulation. Compared with N inefficient genotypes, N efficient genotypes had more obvious advantages in N absorption rate, the amount and the percentage of N accumulation during the growth phases from elongating to heading and from heading to maturing. And of which the advantage during the growth phase from heading to maturing was particularly significant. Since rice organs aged and the plant physiological functions declined gradually from heading to maturing, the characteristics of rice senescence was then studied to understand the relationships between rice senescence and the N absorption and accumulation, and the N use efficiency. Results showed that, during the period of grain filling, the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in flag leaf of N efficient genotypes were obviously higher than those of N inefficient genotypes. Contrarily, content of malondialdehyde (MDA), which is the product of membrane lipid peroxidation, was lower in N efficient genotypes than in N inefficient genotypes significantly. After the stage of full heading, N use efficiency was positively correlated to the activities of SOD, POD, and CAT in flag leaf of rice and negatively correlated to the content of MDA. So it reveals that, at late growth stages of rice, the activities of SOD, POD, and CAT of N efficient genotypes were higher than those of N inefficient genotypes in flag leaves, which could eliminate reactive oxygen species, inhibit the membrane lipid peroxidation and decrease the content of MDA more efficiently in N efficient rice genotypes. Therefore, compared with the N inefficient genotypes, the leaf senescence of N efficient genotypes was relatively slower, which could prolong the functional period of leaves, enhance the organic matter accumulation and improve the efficiency of N absorption and utilization.