Leaf Area Index (LAI), as a fundamental parameter to evaluate the physiological condition of plants, was calculated by image processing based on machine vision technology. The measurement system hardware consisted primarily of the MS3100 3CCD camera, the image grabber card, a desktop computer and the acquired images were processed by Matlab and ENVI. After acquiring the 3 images by the 3CCD camera of Green, Red and NIR channels, the NIR image was considered more effective in separating the soil background for its higher contrast value. Thus, it was selected for image processing to calculate the leaf area index (LAI). The transect method was applied to obtain the threshold 50 in the binary image conversion and the soil background was thus eliminated as a result that most of its reflectance in the image was under 50. Then the 'imerode'-'imdilate' operation in the image processing box of Matlab was used to remove the left crop stem noises, including those small weeds in the binary image background. Consequently, the LAI of the acquired NIR image was calculated as 0.523 by dividing the total image pixel amount by that of the black pixels in the binary image.