Abstract: Objective To study the set-up margin of clinical target area by analyzing the set-up errors of cone-beam CT in 16 lung cancer patients undergoing fractioned radiotherapy. Methods Sixteen patients with lung cancer（8 with left lung cancer,5 with right lung cancer and 2 with mediastinal lymphanode metastasis） who underwent image-guided radiation therapy（IGRT）were included in this study.All patients were fixed with thermoplast phantom and underwent IGRT every day.Set-up errors of cone-beam CT were calculated according to its matched and planned CT images in left-right（x）,superior-inferior（y） and anterior-posterior（z） directions. Results The set-up error of cone-beam CT in the x,y and z axes was（0.11±0.06）cm,（0.14±0.10）cm and（-0.04±0.06）cm respectively,while the set-up error of cone-beam CT in its rotary axis was 0.57°±0.17°,0.05°±0.20°and-0.01°±0.33°,respectively.The set-up error of cone-beam CT in the y direction was relatively larger than in the x and z direction. Conclusion Application of one-beam CT can significantly reduce the set-up errors of radiotherapy in patients with lung cancer and the effect on PTV margin with an outward extension of 0.62cm,0.82cm and 0.49cm in the x,y and z directions,respectively.