Nitrogen use efficiency (NUE) is one of the most important physiological traits related to low N tolerance. The present investigation was carried out during 2016 and 2017 seasons at the Agricultural Research and Experiment Station, Faculty of Agriculture, Cairo University, Giza, Egypt. The objectives were to study the effect of low N level on NUE, yield and grain composition as well as to determine the relationship between physiological traits (N accumulated in grain, Ng; NUE and grain produced per unit of grain N, Gw/Ng) and grain yield/plant (Gw). Two single cross hybrids were evaluated under three N levels, namely low N level (216kg N/ha), control (288kg N/ha) and high N level (360kg N/ha). Therefore, the amounts of applied nitrogen in the soil were 3.75, 5.0 and 6.25g/plant under low N, medium N (control) and high N, respectively. A split-plot design in a randomized complete block arrangement was used with four replications. The percent change relative to control showed that high N level caused a significant increase in grain yield/plant, grain yield/ha, kernels/row, ear length, N% in grain, Ng and grain protein % by 13.8, 14.53, 9.12, 6.99, 8.25, 22.85 and 7.2%, respectively. On the other hand, low N level caused a significant increase in NUE by 27.53% and Gw/Ng by 3.70%. Results of the simple regression analyses revealed that the superior traits in their contributions to Gw were NUE where R2 values were 0.99, 0.98 and 0.96 along with Ng where R2 values were 0.95, 0.66 and 0.94 under low N, medium N and high N, respectively.