Simple, accurate and non-destructive models determining leaf area of plants are important for many experimental comparisons. Determining the individual leaf area (LA) of hazelnut (Corylus avellana L.) involves measurements of leaf parameters such as length (L) and width (W), or some combinations of these parameters. Two-year investigation was carried out during 2005 (on twenty genotypes) and 2006 (on one cultivar) under open field conditions, respectively to test whether a model could be developed to estimate leaf area across genotypes and environments. Regression analyses of LA vs. L and W revealed several models that could be used for estimating the area of individual hazelnut leaves. A linear model having LW as the independent variable (LA = 2.59 + 0.74 LW) provided the most accurate estimate (R2 = 0.982, MSE = 29) of hazelnut LA. Validation of the model having LW of leaves measured in the 2006 experiment coming from other genotype grown under different environmental conditions showed that the correlation between calculated and measured areas was very high.