Analysis of plant leaves image so far been an important and difficult task, especially for leaves with complicated background where some interferents and overlapping phenomena may exist. To extract the plant leaf from complicated background is challenging since we need to remove the soil, residue and etc. In order to obtain the greeness of image, Excess Green Vegetation Index is obtained to ease the segmentation process. But, the problem raise when dealing with uneven illumination. This paper proposed Modified Excess Green Vegetation Index to overcome this problem. The conventional based-auto-threshold method (Otsu's thresholding), ExG (Excess Green Vegetation Index) was compared with Modified Excess Green Vegetation Index to evaluate the performance of them. Experimental results have showed that Modified Excess Green Vegetation Index has superior performance over ExG Otsu's threshold in term of is insensible to illuminant variations.