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Roundness error is a shape error that affects the performance of mechanical parts, especially rotary motion performance. Its measurement generally needs to be carried out in the normal section of the work-piece. Articulated coordinate measuring machine is a manual in-situ measuring instruments widely applied in production line, which cannot execute measurement strictly in a cross-section. This paper reports an algorithm for evaluating the roundness error from the measured data by articulated coordinate measuring machine. Firstly a cylindrical surface is fit from the measurement data that disperses on the surface of the mechanical part under test. Secondly an evaluation plane is selected according to least squares method. The roundness error is calculated through projecting the measured points into the plane and in conformity with least square method. The evaluation result is verified by comparing with the result given by a Descartes coordinate measurement machine. Furthermore, the contribution of various measured points to the roundness error is estimated by way of region division. Experimental results show that the evaluation method is reliable and effective.