In this study, we proposed and evaluated position correction accuracy assessment method with a phantom for IGRT system with add-on six-degrees-of-freedom radiotherapy (6D) couches in couch rotation. Methods and Materials: A phantom was used in a self-build phantom. We were scanned with computed tomography (CT) for radiotherapy planning and planned treatment isocenter to fall in line with CT center by treatment planning system. At first, we examined data of CT slice thickness for digitally reconstructed radiograph of QA phantom. Next, we measured uncertainty for IGRT system. We performed position correction accuracy for IGRT system with QA phantom and digital angle meter. Results: Detection and correction errors for pitch and roll direction were within 0.3 degree in all verifications. Conclusions: We proposed a quality control method for position correction accuracy of 6D couch. The method was able to evaluate the accuracy of detection and correction of 6D couch and revealed the deviation of the origin of the couch rotation.