Abstract: Objective To evaluate set-up errors at the directions of right-left(X), anterior-posterior(Y) and superior-inferior(Z) of thoracic tumors treated with intensity modulated radiotherapy(IMRT) by simulator. Methods One hundred and ninety patients with thoracic tumors, 150 males and 40 females, were treated with IMRT in the prone position and verifi ed with simulator. Position verifi cation was performed once in 153 patients and twice in 37 patients. Patients were detected by enhanced computer tomography after vacuum pad fi xing. Then Digital reconstructed radiographies (DRR) of 0, 45 and 90 degree were generated. DRR of each patient was compared with portal images and set-up errors were recorded and analyzed. The shift margin from internal target volume(ITV) to planning target volume(PTV) was calculated according to set-up error data analysis. Results The mean and standard deviations of set-up errors of all patients at the directions of right-left (X), anterior-posterior (Y) and superior-inferior (Z) were (1.3±1.8)mm, (1.3±1.9)mm and (1.7± 2.3)mm, respectively. The maximum set-up error at the directions of X, Y and Z was 7mm, 7mm and 8mm respectively. And the margins from ITV to PTV were 3.9mm, 3.9mm and 5.0mm, respectively. The mean and standard deviation of set-up errors of male and female patients at X, Y and Z directions were (1.2±1.8)mm vs. (1.6±1.9)mm(P=0.18), (1.3±1.8)mm vs. (1.4±2.0)mm(P=0.50) and (1.6±2.2)mm vs.( 1.9±2.5)mm(P=0.81), respectively. Conclusion It was recommended that shift margin from ITV to PTV should be calculated according to set-up error data. The effects of respiratory activities on the target area could be observed under posture validation at simulator, which provided reference during GTV to generate ITV.