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Leaf area index (LAI) is an important variable in agronomical research for crop monitoring.Indirect optical instrument LAI-2000 was used to estimate LAI in lowland irrigated rice (Oryza sativa L.), and LAI based nitrogen (N) diagnosis was discussed.In order to enlarge the application of LAI based N diagnosis model, a simple deterministic model was modified to determine the LAI in the field.Two field experiments were conducted in Jiangsu Province in East China from 2013 to 2014.The comparison of LAI-2000 records and LAI offered a conversion equation (LAI =1.6772*LAI-2000-0.6652, R2 =0.803**, n =90) for LAI-2000 application in practical use.The linear regression analysis showed that there is a significant relationship existed between NA and LAI, and a diagnostic model (NA =29.384*LAI-3.5141, R2 =0.845**, n =300) was developed for N diagnosis in lowland irrigated rice.Finally, plant characters were combined to fit LAI, and plant height (H), tiller number (TN), leaf length (L) and width (W) were used to modify a simple model (LAI =0.3971*(H*TN*L*W)/1000 + 1.0167, R2=0.885**, n =25),which enables the farmer to obtain the LAI information occurring within his rice field rapidly and non-destructively.