Grapheme-to-phoneme (G2P) conversion plays an important role in speech processing applications and other fields of computational linguistics. Sinhala must have a grapheme-to-phoneme conversion for speech processing because Sinhala writing system does not always reflect its actual pronunciations. This paper describes a rule based G2P conversion method to convert Sinhala text strings into phonemic representations. We use a previously defined rule set and enhance it to get a more accurate G2P conversion. The performance of our rule-based system shows that the rule based sound patterns are effective on Sinhala G2P conversion.