Leaf area index is one of the most important parameters in ecological and environmental studies. This paper presents a method for single leaf area measurement based on the counting of the leaf pixels in digital leaf image. Initially, the target leaf is put on a piece of white paper on which four endpoints of an equal-length and orthogonal cross are printed. A background color threshold is then set to separate the white background pixels from the leaf pixels. After the removal of the background pixels, the leaf pixels are left; and the ratio of the number of leaf pixels to the number of pixels of the reference facet formed by the four control endpoints is equal to that of their areas. The single leaf area can then be computed easily based on this proportional relation. Analysis and experimental results indicate that the proposed method is an efficient and precise method for single leaf or leaf-like object area measurement.