The effects of nitrogen fertilizer rate on carbon-nitrogen metabolism and nitrogen use efficiency of summer maize (Zea mays L. cv. Zhengdan 958) were studied by 15N labeling under high and intermediate yield conditions. In the high-yield field, proper nitrogen fertilizer rate increased gain yield, but excessive nitrogen fertilizer rate did not have further yield-increasing effect and nitrogen use efficiency was low (29%). In the medium -yield field, grain yield increased and nitrogen use efficiency decreased with the rise of nitrogen fertilizer rate. The order of 15N accumulation in different organs was: grain, leaf lamina, stem, root, and leaf sheath and spike axis. In the high-yield field, accumulation of 15N in grains and leaves declined, but increased in stems and roots, when nitrogen fertilizer rate was over 300 kg·hm-2. In the medium-yield field, the accumulation of 15N in grains and spike axes increased with the increase of nitrogen fertilizer rate. The highest activities of nitrate reductase, glutamine synthetase, and sucrose phosphorate synthetase in leaves and sucrose synthetase in grains were observed in the high-yield field with a nitrogen fertilizer rate of 300 kg·hm-2. When nitrogen fertilizer rate reached 400 kg·hm-2, there is no further increase in the activities of abovementioned enzymes. However, the activities of these enzymes increased with the increment of nitrogen fertilizer rate in the medium-yield field.