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Plant-based diagnosis is important for determining the status of crop nitrogen (N) nutrition. We aimed to localize a critical N dilution curve and validate the applicability for maize hybrids in the North China Plain (NCP), and to explore the feasibility of using leaf chlorophyll (Chl) to estimate N nutrition index (NNI). Here, the use of a gradient of N fertilizer treatments with maize hybrids was conducted for 2 years, and Chl content from the youngest collared leaf in addition to NNI values were determined for the first season. The parameters of the fitted critical N curve were essentially identical across hybrids and years. The localized curve (Nc = 33.395 W−0.396) and NNI could strongly distinguish plant N status. Chl and relative Chl (R-Chl) increased with increasing N rates, and both were significantly positively related to NNI. Genotype influence was not significant when Chl was normalized to R-Chl, and the response curves of NNI as a function of R-Chl were more stable between the two hybrids. The relationships between relative yield and NNI and R-Chl were expressed as quadratic functions, but the slopes/intercepts varied across hybrids and stages. Therefore, the localization and validation of the critical N dilution curve for summer maize in NCP was confirmed, there were no differences among hybrids and climates, and the R-Chl index was suggested as a candidate for estimating NNI under this conditions. However, further investigations are needed before these results are applied to practical production. [ABSTRACT FROM AUTHOR]