Thai criminal law has long been developed and can be divided into three periods. The first period was before the 20th century. In this period, the old style of law played a crucial role in inflicting criminals. The second period actually began under Chulalongkorn's reign in the 20th century during the time of modernization. In this period, criminal law was codified and western criminal law was imported and mixed with old legal principles by foreign law experts to be freed from extraterritorial rights given to many countries and to be important tool for country's development. However, since Thailand later switched its regime from absolute monarchy to democracy and situations changed in many aspects, the new penal code was drafted and promulgated in 1958. This legal code generally received concepts from the old one and imported the new idea to make the new code modern. Since then, the Criminal Code was amended about 30 times without major change. It demonstrates that Thai Criminal Code still works well. However, some concepts should be taken into consideration so that the Code can serve justice for all people who are involved.