Abstract: It is a critical operation step for manufacturing elbow pipe to make tangential plane curve on the round pipe. An easy-to-use marking-off method is introduced, which involves no need of any template, but only requires few steps, i.e., first identifying all the prime line length-points of the elbow circumfere nce bisector by means of rough calculation, then marking the points on the round pipe and connecting them to get the desired tangential plane curve based on which elbow with any angle and any diameter can be made via cutting and pairing.