Technical Abstract: High-precision and low-cost electronic diagnostic devices are needed for monitoring crop nitrogen status and variable rate fertilization. One normalized difference vegetation index portable sensing device was designed in this study based on the peak red wavelength of 650 nm and the peak near-infrared wavelength of 940 nm. A revised normalized difference vegetation index formula was developed based on optical absorption and transmission principles and through extensive experiments, and a prototype of the device with both hardware and software was constructed. Comparison experiments using corn leaves showed that the device provided comparable measurement results with a SPAD-502 chlorophyll meter. The revised normalized difference vegetation index formula proposed in this study will be useful for other applications and the portable sensing device designed will provide a useful tool for real-time, non-damage monitoring of crop nitrogen status and for the implementation of variable rate fertilization in precision agriculture.