In medieval law of Thailand country. the Ayutthaya period has a biblical Scriptures treatise of Justice as a legal article that came from India which define the criteria. Conduct for the general public, including the king. Thammasat Scriptures have characteristics comparable to constitutional law itself. The King is in the Scriptures is a legal treatise, seal up and there has been significant improvement of the law, it is the legal instrument is adjusted to samduang, which is a compilation of laws and legal classification of the. The legal instrument that has samduang the law as a legal act, contracts, inheritance, family, criminal, so it will be seen that in the Ayutthaya period has already taken the major Scriptures of the treatise and the improvement of legal instrument with samduang.