Abstract: Objective To assess the value of CT and MRI registry in the protection of optic chiasm in 3 dimensional conformal therapy of head and neck tumor. Methods Sixteen cases of head and neck tumor were performed 3 dimensional conformal therapy.The marker was mixed liquor produced by iohexol and Gd-DTPA (1.61 mg/ml) with volume ratio of 6∶1.The marker was fixed on the facemask as the sign of the CT and MRI registry.Scanning parameter of CT was the same with MRI.If three markers were displayed in the same slice both on the CT and MRI,that indicated successful registry.The optic chiasm was drawed on the CT and MRI respectively. Results The markers on the facemark were clearly displayed with no artifacts.The optic chiasm were clear displayed on MRI in all 16 cases:“×” shape in 3 cases,“-” shape in 13 cases,“” shape in 2 cases and “” shape in 1 case.The CT scan showed the optic chiasm was clear in 11 cases:“-” shape in 3 cases and “” shape in 8 cases.The optic chiasm was not been seen in 5 cases. Conclusion The mixed liquor produced by iohexol and Gd-DTPA (1.61 mg/ml) with volume ratio of 6∶1 was good marker in CT and MRI registry.CT and MRI registry can display the optic chiasm clearly,which was useful to the protection of optic chiasm in 3 dimensional conformal therapy of head and neck tumor.