Abstract: Breeding selection of high N efficient cultivars is an effective way to increase nitrogen (N) fertilizer efficiency and reduce N loss in arable fields. Nevertheless, the agronomic basis underlining it is still quite vague. Root size and N uptake system are two components determining N accumulation. Solution culture study has already proved the importance of root on N accumulation. However, both root growth and N accumulation are dynamic processes at field conditions. It is not clear how the root growth affects N accumulation and yield formation under different N levels. In this paper, 4 (hybrid) maize cultivars (2 N-efficient and 2 N-inefficient) were adopted to investigate the relationship between root growth and N uptake as well as yield formation under two N levels (with or without N supply). Results showed that N efficient hybrids (NE1 and ND108) took up more N than inefficient hybrids. The difference on the N uptake was mainly attributed to N accumulation after silking, while difference of N accumulation before silking stage was not significant. High N efficient cultivars, NE1 and ND108, had larger root dry weight at both N levels. However, under low N (without N application), 209115, one inefficient hybrids, had almost the same root weight as ND108 despite the lowest N accumulation and yield. There is a close correlation between post-silking root size and N uptake during the whole growth period. In (N-efficient) hybrids, efficient N uptake before silking helps the build up of a big root system which, in turn, lead to a big N uptake after silking. As the results, N export from leaves was reduced and a longer photosynthetic activity was maintained. The result of this study proved that root size was the dominant factor determining N accumulation; nevertheless, the role of N uptake rate should not be neglected.