A method for measuring parameters of plant leaf is proposed using image processing software developed with Visual C++6.0 based on image processing techniques and a digital scanner. Images with plant leaves and reference objects are acquired using a digital scanner. Then, image segmentation, component labeling and contour extraction are operated and the image parameters(including area, perimeter, length and width) of the leaves and the reference objects in these images are obtained. According to the real parameters and the image parameters of the reference object, two calibration factors can be gained. Finally, the real parameters of plant leaves are detected with the calibration factors and the image parameters of plant leaves. Measuring experiments on three kinds of leaves are performed and experimental results compared with a typical manual method demonstrate that the the proposed method in this paper is efficient and practical.