Herpes Simplex Encephalitis is associated with a high mortality rate and a high probability of neurological sequelae. Good results are obtained when Herpes Simplex Encephalitis is promptly diagnosed and treated with antiviral agents. We reported a case of Herpes Simplex Encephalitis in which Magnetic Resonance Imaging (MRI), Single Photon Emission Computed Tomography (SPECT) and Fluorodeoxyglucose-Positron Emission Tomography (FDG-PET) were performed sequentially. Imaging studies are valuable tools for early detection and diagnosis of Herpes Simplex Encephalitis. And particularly FDG-PET represents functional anatomy through diffuse decline of glucose metabolism better than other imaging studies.