Abstract: Forest site quality, the capacity of an area to produce tree growth, is very important, for forest management and silviculture. The interpretation of forest site from aerial photographs will generally be made in terms of topography, soils or vegetation. But we have studied testing the use of method for evaluating site quality by quantitative factor in the experiment of Yubaiding Yunnan pine forest area, Yunnan province. This experiment was carried out by using the site factor that is measurable on aerial photographs. Even though the tree age cannot be measured on aerial photographs, however, the height and crown diameter of the trees can be measured. The greater the tree height/the crown diameter ratio is, the better the forest site quality is Under Yunnan pine(Pinus yunnanensis) species condition. Therefore, the ratio of tree height to crown diameter will provide an evaluation of site quality. Besides, the stand density has been taken into account in evaluating site quality by this method. The method is available in practice. The advantage of the technique lies in the fact that can raise the efficiency of work by comparing to the conventional work.