Objective To observe CT findings of brain malignant tumor treated by X-knife. Methods 59 patients of brain malignant tumors after the treatment of X-knife were underwent plain and enhanced CT scan.According to the rate of disappearance,the density,the changes of edema and the radioactive encephalitis,all patients were followed up. Results Metastatic brain tumor disappeared faster than primary ones,the more extensively primitive tumors in metastasis disappeared more rapidly.Lymphoma and germinoma disappeared faster than astrocytoma.The low density tumors did not recured,the middle density tumors without enhancemant recured difficultly and the blend density tumors with middle enhancenent recured easily.Conclusion Plain and enhanced CT scan can be a good standard to demostrate the effect and prognosis of brain malignant tumor after X-knife treatment.