Abstract: A new method of detecting roundness error of cylindrical inner-hole based on coaxial hollow laser technology was presented. In view of the optical properties of cone lens and structural characteristics of cylindrical inner-hole, a detecting optical system which permits to reflect the detecting beam only once was presented. In the experiment, the roundness error of the cylindrical part was tested, and the test data was analyzed with the least square method. The result shows that the detection accuracy can reach 0.1 m. The method shows the advantages of optical detection technology such as non-destructiveness, noncontact, high precision and thus it can be used to detect the morphology and position error.