Abstract: Nitrogen(N) fertilizer use efficiency can be improved by two ways,that is,optimized N fertilizer management and genetic improvement of nitrogen use efficiency(NUE) of crops.In this paper,the biological mechanism of NUE is analyzed and the biological characteristic of N-efficient maize genotypes is proposed.It is suggested that,under N limitation conditions,a N-efficient maize genotype should be,(1) before flowering,a steady N uptake should be maintained the N should be utilized efficiently for ear development to enhance floret set and,therefore,a big sink is established for carbon and N accumulation during kernel filling stage.Root growth at this stage should vigorous,a trait essential for N uptake at late stage.Leaf expansion should fast so that a big canopy is established.(2) after flowering,N(especially the mineralized soil N) is taken up efficiently by the pre-established big root system,and is used for kernel growth.By this way,N export from leaves is slowed and leaf photosynthetic rate is maintained.As a result,more photosynthate is available for kernel filling.It is stressed that a big ear,a large root system and stay-green leaves are most important secondary traits for breeding for N-efficient genotypes.