According to JIS (Japan Industrial Standard), definition of positioning accuracy about the robots is clearly described but the measurement method of the accuracy is not described concretely. In this study a measurement method of positioning accuracy using the reference plate with grid patterns and the image processing probe with CCD camera is proposed. This measurement procedure consists of two steps. In the first step, the positions of grids on the reference plate are measured by the probe mounted on the 3-D coordinate measuring machine. In the second step, the robot is commanded to move at each position of grids on the plate placed in the robot's working area and then positioning accuracy about the robot is measured by calculating the difference between commanded position and performed position. The detective accuracy for certain grid is within 0.6 μm in standard deviation and that for 9 grid patterns in the 400×360 mm area is within 3 μm. And this method is actually applied to a SCARA robot.