Abstract: The ‘3414’ field experiment on nitrogen and phosphorus fertilization were carried out in a corn field in Chongqing, China. Nitrate-nitrogen, ammonium-nitrogen and extractable phosphorus of leaves and sheaths at the third and sixth places of fresh corn were determined rapidly by using a portable high-strength photometer at the sites. The results indicate that there are significant differences between nitrate-N, ammonium-N and extractable P contents of leaves at the same place and those of sheaths, respectively. The differences between the N and P contents of leaves at the same part of the different places are not accordant. The differences of nitrate-nitrogen, ammonium-nitrogen of leaves and ammonium-nitrogen of sheaths between the third place and the sixth place are not significant. While the differences of extractable phosphorus of leaves and sheaths and nitrate-N of sheaths between the third place and the sixth place are all at 1% significant levels. Nitrate-nitrogen, and ammonium-nitrogen of every part of leaves at the different places are positively correlated with the N application rate and total N, respectively. At the same time, extractable phosphorus of every leaf part at the different places is positively correlated with the P application rate and total P. And the relativity of sheaths at the third place is the best, and all the correlations are at 1% significant levels. The optimal part for determining nitrate-N, ammonium-N and extractable P at the same time is sheaths at the third place. Nitrate-N is better than ammonium-N to diagnose nitrogen nutrition of corn from theregent costand content level of soil nitrogen.