Technical Abstract: The ability to determine the minimum critical nitrogen (N) content in maize plants in order to obtain maximum growth is important to the management of the crop. To conduct plant-based N nutrition diagnosis in maize, it is essential to establish a critical N concentration ([N]), which is the minimum nitrogen concentration necessary to achieve maximum growth. Several authors have shown that [N] declines as a function of aerial biomass accumulation (W): [N] = 3.4W raised to the power of -0.37. This [N] – W relationship has been shown to be a promising diagnostic tool of nitrogen status in maize. However, when applying this [N]-W relationship in maize nitrogen status diagnosis, knowledge on both biomass and nitrogen concentration of the whole aerial part are needed, making the diagnosis process both labor intensive and time consuming. The goal of this study is to evaluate the applicability of relating chlorophyll meter readings with the critical [N] to provide a tool for whole plant nitrogen status diagnosis in maize. Biomass of shoot and specific organs, nitrogen concentration, as well as chlorophyll meter measurement of specific leaves, were determined bi-weekly during the growing season in 2007 and 2008. Another set of data collected in 2005 was also used in validating the relationship between chlorophyll meter measurement and leaf nitrogen concentration. The measured [N] of the whole plant followed the trend described by the [N]-W relationship well. Nitrogen concentration measured from different organs indicated that whole plant nitrogen status can be represented by the nitrogen concentration of the top most fully expanded leaf. A quantitative relationship between nitrogen concentration and chlorophyll meter measurement on the upper most expanded leaf was established and validated. The established relationship between chlorophyll measurement and leaf nitrogen concentration strengthened the applicability of the critical [N] - W relationship in the diagnosis of maize nitrogen status.