Until the nineteenth century the source of criminal law in the kingdom was ancient Thai law based on the Indian Dharmashastra (a Hindu legal code attributed to Manu), which was introduced into the country during the Ayutthaya era (see The Ayutthaya Era, 1350-1767 , ch. 1). Over the centuries this code was augmented by numerous and sometimes conflicting royal laws and decrees, and there was little uniformity in the interpretations and applications made by different judges. The resulting tangle of legal concepts and arbitrary judicial decisions was strongly criticized by Western countries whose nationals were brought in as advisers or engaged in commerce in the kingdom during the nineteenth century. Objecting to the complexities, cruel punishments, delays, and injustices of the legal system, each Western government insisted that its nationals and others under its protection in the kingdom be subject only to the jurisdiction of its own extraterritorial courts. By the middle of the nineteenth century, the system of extraterritoriality was firmly established and had further complicated an already confusing legal structure.
Concerned by the limitations on the country’s sovereignty and encouraged by treaty promises that extraterritoriality would be ended when the laws and judiciary were modernized, the Thai rulers set about making legal reforms. Whereas earlier kings had attempted to codify existing law and eliminate many of the harsher punishments, King Mongkut upon his accession to the throne in 1851 went further. Proclaiming the equality of all people before the law, he tried to improve standards of judicial honesty and competence and to abolish the delays and conflicting rules that had become so much a part of the judicial administration .
During the reign of King Chulalongkorn, who succeeded Mongkut in 1868, legal reform took a new direction. Whereas previous kings had tried to revise and adapt ancient law to meet modern needs, Chulalongkorn believed that the problem would be solved not by revising the old system but by replacing it. He created the Ministry of Justice in 1892, extending its powers to all courts of the kingdom. The ministry’s first task was to develop a modern uniform court structure, a process that continued until 1920, ten years after Chulalongkorn’s death.
During this period existing statutory and customary laws were collected and codified, and an enormous volume of new legislation was added. In 1897 a commission composed almost entirely of French and Belgian lawyers was appointed to draw up a penal code, which was promulgated in 1908. The constitutional monarchy established after the coup of 1932 brought about further legal reforms, promulgated in 1935 in the Criminal and Civil Procedures Code. This new legislation was based on Thai and Western legal practices that provided substantial safeguards in the administration of justice. In response to these legal reforms and the incorporation into Thai law of some Western concepts of jurisprudence , the system of extraterritoriality was completely eliminated by 1938. Further legal refinements resulted in the Criminal Code of 1956, which in the late 1980s remained the core of Thai criminal law.