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