On a linear accelerator for radiation therapy, the radiation source is installed on a gantry that rotates about an isocenter. During radiation therapy, the cancer of a patient is positioned at the system isocenter so that the cancer can be irradiated from different angles as the gantry rotates. As the radiation dose controlling technique improves, more attachments are added to the gantry so that the physical space for positioning a patient becomes smaller and the motion range of the gantry is more restricted. The reduced space creates difficulty for positioning large sized patients. One method for solving this problem is to put the patients away from the radiation source which makes the cancer of the patients not located at the isocenter. This paper describes a method for calculating the patient location to ensure the cancer to be on the path of the radiation beam while not at the isocenter. The method also compensates the vertical position error caused by bending of the patient support couch. The method has been successfully tested on the linear accelerator with a dummy.