Abstract: Canopy images of summer corn were taken in the 6-leaf and 10-leaf stages in a field experiment with six N application levels. And the relationships between color parameter indexes of corn canopy images and N application level, leaf SPAD, stem sap nitrate concentration, plant total N content and nitrate N content in 0~90 cm soil were determined. The results show that in the 6-leaf stage, color parameter indexes of B/(R+G+B), G/B, R/B, B/L are significantly linearly correlated with N application level, leaf SPAD, stem sap nitrate concentration, plant total N content and nitrate N content in 0~90 cm soil (P<0.01). And B/(R+G+B) presents most significant correlation, and B/L is next to it. In 10-leaf stage, the correlations are not so significant as those in 6-leaf stage. The results suggest that it is feasible to use digital image processing technique to diagnose N nutrient statues of summer corn. The 6-leaf stage is crucial period with B/(R+G+B) as the index for the N nitrogen diagnosis using digital image processing technique.