TTS system initially transforms text input into a sequence of sound symbols. The complexity of Thai requires about several hundred of rules including main and specific rules to derive most pronunciations for the rule-based approach. Our proposed rule-based system is based on the syllable structure analysis. To increase the quality of TTS, an exception dictionary to cover anomalous pronunciations and using rule reference engine to determine sentence structure are included in the system. The phonetic generation and speech generation by mapping table are sequential steps to transform a set of phonetic symbols to speech waveform.