Abstract: A new leaf area measuring instrument was developed by using poly-Si photoelectric sensor board and uniform lighting slab with light source systems for the fast, nondestructive and accurate measurement of plant leaf area. Back propagation neural networks were used to develop the relationships between leaf area index and light intensity, leaf transmittance, environmental temperature, voltage values of photoelectric sensor board. The results indicated that this instrument could measure the leaf area with different shape, thickness and softness. The determination coefficient (R2) between the measured and predicted values was 0.98, and the measuring accuracy was 94.8%. It was concluded that a new measuring method and instrument were developed for the detection of leaf area.