Herpes simplex virus(HSV) infections of the CNS are associated with significant morbidity and mortality even when appropriate antiviral therapy is administered. HSV infections of the brain can be subdivided into two categories : neonatal HSV infections, which usually are caused by HSV type 2, and herpes simplex encephalitis(HSE), which occur in patients over 3 months old and is nearly uniformly caused by HSV type 1. The clinical presentation of HSE is one of the focal encephalopathic process associated with altered levels of consciousness, fever, focal seizures and hemiparesis. But because of the lack of pathognomic clinical presentation and diagnostic procedure, the efforts to develop alternative diagnostic procedure have led to the use of new diagnostic technique such as polymerase chain reaction(PCR). We report a case of HSV type 1 encephalitis in 13 month old male infant who presented with altered level of consciousness, fever and focal seizures. With the use of the PCR, HSV-1 DNA was detected in cerebrospinal fluid from the patient. The symptoms and signs of encephalitis subsided by treatment with acyclovir in 14 days.