Abstract: Objective To investigate the feasibility of CT-MRI dual-mode external marker as images registration for brain tumor radiotherapy. Methods The Gd-DTPA diluents with maximum value of the signal intensity in T1WI was mixed with different proportions of iohexol. The signal intensity of T1WI, T2WI and FLAIR in MR and CT value were measured. The intermixture with a right proportion was sealed in a thin plastic pipe as the dual-mode external marker and applied in clinic.Its feasibility was evaluated by QA phantom test.Fifteen patients with brain tumor underwent CT and MR scanning according to radiotherapy planning requirements.The data was transmitted to Varian workstation for Match points registration. Results The diluted concentration of 1.61 mg/ml had the maximum value of the signal intensity in T1WI.The intermixture of Gd-DTPA diluents and iohexol by 6:1 was chosen as CT-MRI dual-mode external marker.The mean error was (1.35±0.22) mm in images registration. Conclusion The intermixture, Gd-DTPA dilution with concentration of 1.61mg/ml and iohexol by 6:1 might be chosen as the dual-mode external marker of images registration for brain tumor radiotherapy.