Abstract: Determination of the critical and optimum values of spectral diagnosis indexes at the different growth stages of rice is crucial for precise fertilization. NDVI values of early rice of different nitrogen rate treatments at key growth stages were measured using active spectrometer Greenseeker. Taken the grain yield as criteria, the optimum NDVI ranges and the critical NDVI values for early rice variety Zhongxuan 181 at key growth stages at Yingtan, Jiangxi province were determined with the discriminant analysis. The results showed that the optimum NDVI ranges at tillering, jointing and heading stages were 0.37-0.55, 0.76-0.80 and 0.72-0.75, respectively. According to the discriminant function, the critical NDVI values at tillering, jointing and heading stage were 0.346, 0.703 and 0.654, respectively. The critical NDVI values were tested using a dependent field experiments with 27 plots, and the correct classification ratios of yield based on the NDVI at tillering, jointing and heading stage were 63.0% (17 out of 27), 81.5% (22 out of 27) and 96.3% (26 out of 27), respectively. The results indicate that canopy NDVI at key growth stages can be used to forecast the yield potential level and as a guideline for fertilization determination. But further studies should be carried out to explore the optimum fertilizer rate for high yield under different NDVI values and establish the fertilization model based on the NDVI.