Field investigation, nutrition diagnosis and chemical analysis were employed to study the relationship between nitrogen rates and nitrate contents in flue-cured tobacco leaves. The results showed that nitrogen rate had remarkable influence on yields and quality, and the contents of total nitrogen and nicotine of cured tobacco leaves. A reverse S model curve of nitrate content in the latest fully expanded leaves was observed, which could be simulated by duality and quartic equation with highly significant relation. The reverse “S” of nitrate contents in tobacco leaves from different fields changed evidently in abscissa, yet little in ordinate. The nitrate content in the latest fully expanded leaves increased with the increasing nitrogen rate, and could reflect the nitrogen nutrition status of flue-cured tobacco plants, so it could be applied in the rapid test of nitrogen nutrition.