This paper presents the measurement of the slide error of a large-scale ultra-precision lathe with an effective fabricating length of 2000 mm. A cylinder workpiece with a diameter of 320 mm and a length of 1500 mm was mounted on the spindle of the lathe with its rotational axis along the Z-direction. Two capacitive displacement probes with a measurement range of 100 μm were mounted on the slide of lathe with its moving axis along the Z-direction. The displacement probes were placed on the two sides of the cylinder workpiece over the horizontal plane (XZ-plane). The cylinder workpiece, which was rotated by the spindle, was scanned by the displacement probes moved by the slide. The X-directional horizontal slide error can be accurately evaluated from the probe outputs by using a proposed rotating-reversal method through separating the influences of the form error of the cylinder workpiece and the rotational error of the spindle. In addition to the out-of-straightness error component, the parallelism error component with respect to the spindle axis, can also be evaluated. The out-of-straightness error component and the parallelism error component of the slide error were measured to be 3.3 μm and 1.68 arc-seconds over a slide travel range of 1450.08 mm, respectively.