Abstract： Objective To investigate and analyze the positioning accuracy of three imaging modalities utilized in image-guided radiotherapy (IGRT):electronic portal imaging device ( EPID),kV portal image (kV planar) and the kV cone beam computed tomography (CBCT).Methods 25 groups of setup errors were simulated on the phantom images through treatment planning system. Digitally reconstructed radiographs (DRRs) were constructed from the CT data which were subsequently used as references to register the EPID and kV planar images acquired at the original position.In addition,the reconstructed 3D-CT images were used to register the CBCT images.Finally,the setup errors using several registration methods were measured to investigate and compare the accuracies of the three imaging modalities used for patient setup.Results 675 groups of residual errors were analyzed.All combinations of imaging modalities and registration method were found to be accurate.The mean residual errors in three directions were less than 1 mm.The method based on grey value match of CBCT images was found as the most accurate with an uncertainty below 0.1 mm.When the manual match was used,the performance of kV planar was more accurate than that of EPID (residual error ＜ 0.65 mm).If automatic registration was applied,kV planar generated similar results as EPID did. Conclusions The three available imaging modalities and their corresponding registration methods are all competent for the clinical application of IGRT in our department.Considering the image quality,radiation dose and the accuracy of registration,CBCT has the priority on 1GRT followed by the kV planar.