Efficient nitrogen (N) utilization is crucial for maintaining grain yield under low N input. Less is known about the role of within-plant N allocation and reallocation on ear development and the factors determining N allocation during the critical period around silking. In this study, two maize hybrids, ZD958 (N-efficient) and LY99 (N-inefficient), were evaluated in a 2-year field experiment under two N rates (60 and 180 kg N ha-1). N transport and allocation into the ear during critical period were investigated using 15N stable isotopic tracer. The number and area of vascular bundles in ear shank, above- and below-ear internode were measured. The two hybrids did not differ in grain yields under high N rate. However, the grain yield of ZD958 was 43.6% higher than that of LY99 under low N rate, deriving from 26.3% and 13.9% increment in grain number and grain weight, respectively. At early critical growth stage before silking, ZD958 increased allocation of soil-derived N to the ear by 225.2% compared with LY99 under low N rate. At late critical growth stage after silking, ZD958 increased allocation of soil-derived N and reallocation of vegetative-N to the ear by 45.5% and 116.6%, respectively, compared with LY99 under low N rate. As a result, ear growth rate and ear N content of ZD958 was 22.2% and 69.1% higher than that of LY99 at the end of critical period. During N allocation and N reallocation, the lower leaves were sacrificed and the N status of the ear leaf and upper leaves was mostly maintained to sustain photosynthesis. In the ear shank, flux rate and N concentration of the xylem sap in ZD958 were 53.1% and 32.5% greater at silking stage, and were 40.8% and 27.5% greater at 14-days after silking, respectively, compared with LY99 under low N rate. Correspondingly, the number and average area of big vascular bundles in ear shank of ZD958 were 56.2% and 31.0% greater compared with LY99. Parameters characterizing the number and area of big vascular bundles were positively correlated with N allocation and grain yield, while that of small vascular bundles were negative. It is concluded that efficient N allocation to the ear at critical period is essential for ear growth and the subsequent vegetative-N remobilization, so as to improve low-N tolerance in high-yielding maize hybrids. A superior vascular system around the ear, especially in the ear shank, can enhance N allocation into the ear and could be regarded as a physiological selection trait in maize breeding to improve nitrogen use efficiency.