An online measurement system is developed for the contour error measurement of large cylindrical parts. The system is controlled by single-chip microprocessor + PC. The rotation of the measured parts is driven by two synchronously rotating rollers, and their surface profile is measured through a non-contact sensor. The companion algorithms for evaluating the roundness error are programmed to extract the contour error value of each equinox of the workpiece surface, and the contour error curve is plotted. Theoretical analyses and experimental results show that the system is feasible to carry out the online measurements of large cylinders, and may be of value for various engineering applications.