The legal system remained an amalgam of the traditional and the modern. In several southern provinces, for example, Islamic law and custom were applicable to matrimonial and inheritance matters among the Muslims. A large part of the modern legal system was made up of criminal, civil, and commercial codes adopted from the British and other European legal systems with some modifications borrowed from India, Japan, China, and the United States. Also, an extensive body of administrative law consisted of royal decrees, executive orders, and ministerial regulations.

The judiciary provided for three levels of courts: the courts of first instance, the Court of Appeal, and the Supreme Court. The courts came under two separate jurisdictions. The Ministry of Justice appointed and supervised the administrative personnel of the courts and instituted reform in judicial procedures the Judicial Service Commission, which was responsible for the independence of the courts, appointed, promoted, and removed judges. As a rule, judges retired at age sixty, but their service could be extended to age sixty-five.

The country was divided into nine judicial regions, which were coextensive with the nine administrative regions (phag), in contrast to the four geographic regions (North, Northeast, Center, and South). At the base of the judiciary system were the courts of first instance, most of which were formally known as provincial courts with unlimited civil and criminal jurisdiction. Petty civil and criminal offenses were handled by magistrates' courts, which were designed to relieve the increasing burden on provincial courts. Offenses committed by Thai citizens on the high seas and outside the country were tried before the Criminal Court in Bangkok. Labor disputes were adjudicated by the Central Labor Court established in Bangkok in 1980. Offenses by persons under eighteen years of age were referred to the Central Juvenile Court and its counterparts in several regional centers.

The Court of Appeal in Bangkok heard cases from all lower courts (except the Central Labor Court) relating to civil, juvenile, criminal, and bankruptcy matters. At least two judges were required to sit at each hearing. Cases of exceptional importance had to be heard by plenary sessions of the court. The appellate court could reverse, revise, or remand lower court decisions on questions of both law and fact.

The Supreme Court, which was the highest court of appeal, also had original jurisdiction over election disputes. Although decisions of the court were final, in criminal cases the king could grant clemency. A dispute over court jurisdiction was settled by the Constitutional Tribunal.

Data as of September 1987