We report a case of primary central nervous system vasculitis (PCNSV) mimicking a cortical brain tumor. A 25-year-old woman presented with a 2-week history of headache and transient right hemiparesis. Brain magnetic resonance imaging (MRI) revealed a cortical-involving lesion on the left frontal lobe. The 6-cm sized lesion showed low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. The lesion had continual linear enhancement on the subcortical white matter and leptomeninges. There was no evidence of hemorrhage on susceptibility-weighted images and no diffusion restriction on diffusion-weighted images. The regional cerebral blood volume was decreased on the MR perfusion images, and spectroscopy showed increased lactate and lipid peaks. The symptoms were aggravated by fever and seizures. Biopsy was performed to rule out tumorous or inflammatory lesions. Pathologically, lymphocytes were infiltrated on the vessels, and the arachnoid membrane was thickened with inflammatory cells. The patient did not have any underlying diseases, including immune disorders. After high-dose steroid administration, her symptoms improved. Two months later, brain MRI showed a reduction in the infiltration of the T2 hyperintensity lesion with subtle subcortical enhancement. We present a case of PCNSV involving the left frontal lobe, showing vasogenic edema, mass effect, and subcortical linear contrast enhancement without hemorrhage or infarction.