Compared to positioning repeatability, the “absolute” positioning accuracy of an industrial robot is often significantly worse. In this paper, we propose a method to measure the 2D absolute positioning error of a SCARA robot. Over the given laser line, the linear positioning deviation and the straightness deviation are measured by using a laser interferometer and a position sensitive detector (PSD), respectively. Then, multiple laser lines are set up by using an optical square such that the parallelism or the squareness to the original line can be ensured. By similarly measuring linear positioning and straightness deviations over these laser lines, the robot’s 2D positioning error can be visually represented as a two-dimensional error map.