A technique for calibrating the profile error of the probe sphere of a contact-type displacement probe for on-machine measurement of aspherical profiles in nanometer accuracy was proposed. A sphere workpiece with unknown profile error is used as the sample for calibration. The workpiece, which is mounted on the machine spindle and is rotated by the spindle, is measured by the displacement probe. At each position of the probe, the profile of the workpiece over one rotation are sampled by the probe. An average of the data removes the influences of the workpiece profile error and the spindle error, resulting in on accurate measurement of the profile error of the probe sphere. Calibration of the whole probe sphere can be realized through moving the probe over the workpiece. Experiments of on-machine profile measurement of an aspherical lens were carried out after calibration of the probe sphere.