Since chili pepper plant size directly reflects the state of plant growth, a method for pepper measurement of plants size was discussed here. Pepper plants were shot from above once per week in the greenhouse since being field planted in spring. The method of processing the pepper plant images was studied, in which the image segmentation of combination of color space and the image morphological operations were applied. And the major axis and minor axis of pepper plant, for describing the size of the plant, were calculated from single connected component in the image being processed. According to the method, a program for pepper plant size measurement based on MATLAB was developed. Experimental results have demonstrated that the method is more reasonable and accurate than artificial measure.