Abstract: Objective This study is to quantitatively analyse the set-up error in the different positions of the irradiated patients using electronic portal imaging device(EPID)to provide the set-up accuracy in the different positions and to understand the acceptance between different oncologist.Methods From January 2006 to December 2007,58 patients were involved in this study,who randomly selected from the different oncology treatment group by different positions(20 patients in the head and neck,19 patients in the chest,19 patients in the abdomen and pelvic).The treatment of all cases were implementated by Elekta Precise accelerator,scanned by Philips AcQ-Sim CT and/or simulated by Nucletron Simulix-HQ simulator,planned by ADCC Pinnacle3 7.4 system.Treatment planning computed tomography(CT)scans were used to create orthogonally digitally reconstructed radiographs(DRRs)for portal image registration,which were compared with portal images from an electronic portal-imaging device(EPID)of Elekta′s iView GT by comparing bone structure in horizontal and vertical directions.Results A total of 102 data were reviewed,the set-up error of horizontal position is 1.02±1.15mm(range 0-5mm),the vertical position is 1.11±1.20mm(range 0-6mm).More than 90% setup error is less than 3mm.The smallest setup error in the horizontal direction and vertical direction is 0.64±0.65mm and 0.91±0.98mm respectively with head or head and neck fixed.The set-up errors were different between the different oncologist.Conclusions The majority of setup errors is in the permissible range.Need to improve the recognition of the EPID in the chest,abdominal pelvic anatomy.The implementation of the X-ray simulator before treatment is an effective means to reduce setup error.