The measurement uncertainty analysis is carried out to investigate the measurable dimensions of cylindrical workpieces by the rotary-scan method in this paper. Due to the difficult alignment of the workpiece with a diameter of less than 3 mm by the rotary scan method, the measurement uncertainty of the cylindrical workpiece with a diameter of 3 mm and length of 50 mm which is measured by a roundness measuring machine, is evaluated according to GUM (Guide to the Expression of Uncertainty in Measurement) as an example. Since the uncertainty caused by the eccentricity of the measured workpiece is different with the dimension changing, the measurement uncertainty of cylindrical workpieces with other dimensions can be evaluated the same as the diameter of 3 mm but with different eccentricity. Measurement uncertainty caused by different eccentricities concerning the dimension of the measured cylindrical workpiece is set to simulate the evaluations. Compared to the target value of the measurement uncertainty of 0.1μm, the measurable dimensions of the cylindrical workpiece can be obtained. Experiments and analysis are presented to quantitatively evaluate the reliability of the rotary-scan method for the roundness measurement of cylindrical workpieces.