HSVI is one of the common agents recurrent aseptic meningitis and fatal endemic encephalitis with an incidence rate 7-10.5% .Routine and available methods of diagnosing Herpes viruses, can^, t diagnose Herpes contamination in virus culture promptly and accurately and because of limitation in frequent methods, molecular new methods such as polymerase PCR chain reaction for recognizing parts of DNA genes of Herpes simplex specially compliment fixed sequence are developed. ). This study for the first time with PCR technique for diagnosing Herpes simplex virus type I & II  in Iran was modified and used. This technique is one of the In this study PCR reaction using a pair of designed special primer for a common area DNA polymerase coding  gene of HSV À & HSV ÀÀ viruses on the DNA achieved from standard species , was modified and studied due to sensitivity and specificity .Multiplying product was cloned and its sequence was determined . The people under study in this research was contained 184 CSF samples suspicious to cerebral encephalitis in children 1-10 years old admitted in Tehran Mofid hospital .After making sure of sensitivity and specificity  of modified test the people under study were analyzed by PCR test .PCR modified product in this study was 454 bps .The sensitivity of test about 50 copies of target DNA gene was determined .This reaction has a very high specificity so that with DNA of many microorganism , crossed reaction was not seen .From 184 samples of CSF , 45 was positive by PCR and in this order incidence of cerebral encephalitis resulting from serotype À & ÀÀ of Herpes virus was about 24 % among the people under study .Achieved result in this study shows the point that this modified molecular method is an effective tool for immediate diagnose of Herpes simplex virus in affected samples .