Abstract: An approach based on cross ratio to distance measurement for a single uncalibrated image is presented.Without any prior knowledge of the camera internal parameters(such as focal length and aspect ratio) or its pose(position and orientation),we put foreword and employ nested cross ratio to compute the length of a line segment on plane surface,while only the length(s) of the certain segment(s) is/are known.Based on previous steps one conclusion can be acquired: Once the lengths of rectangle two sides on one plane surface are known and the corresponding vanishing line of the plane is identified,then the length of any line segment on the plane surface can be measured.The extension of our conclusion is also discussed.