Abstract: Using the typical winter wheat(Triticum aestivum) varieties with high(H),middle(M) and low N(L) efficiency selected in our previous research as test materials,the photosynthetic carbon assimilation characterization in flag leaf and the underlying physiological mechanism were studied under field conditions.The results showed that under deficient N condition(-N),the highest grain yield and nitrogen use efficiency were observed in the variety with H,followed by M,and L.So were the photosynthetic rate(Pn),metaphyll conductance(Gm),carbonic anhydrase(CA) activity,soluble protein content(SP) and RuBPcase activity of flag leaves.However,no regularity was found for the chlorophyll content(Chl),stomatal conductance(Gs),Ca2+-ATPase activity and Mg2+-ATPase activity among the tested varieties with different N efficiency,indicating that the higher conductance capability in the liquid phase of CO2 and higher ability of dark reaction in photosynthesis resulted in a higher photosynthetic carbon assimilation and dry matter production for the high N efficiency variety having under N stress.The leaf source capacity(LSC),period of active duration of photosynthetic rate(PAD),relative steady period of chlorophyll content(RSP),leaf area(LA) and average photosynthetic rate in flag leaf were also the highest in H,then in M and L.Under N rich condition,M had the best performance on grain yield,N efficiency and tested physiological parameters followed by H and L.These results indicated that the regularities in behavior of above-mentioned plant traits or parameters of the tested varieties were not unique under different N conditions.