Competition policy and law is a vital tool to level playing field to all market participants to create free and fair competition, which is desirable for consumer welfare and ASEAN economic integration. This is a rationale behind the ASEAN Regional Guidelines on Competition Policy (ASEAN Guidelines being issued to be a common framework for all ASEAN Member States to develop their national competition policy and law. This study found that there were many areas under the Trade Competition Act B.E.2542, which did not conform to the ASEAN Guidelines, including the inappropriate exclusion for all types of state-owned enterprises, merger control being inapplicable in practice, the imposition of criminal sanction to unfair trade practices and some problems concerning enforcement and due process. However, the Trade Competition Act B.E.2542 was applicable before the introduction of the ASEAN Regional Guidelines on Competition Policy. Therefore, under the new competition law reform the Trade Competition Act B.E.2560 was designed to be basing more on the ASEAN Guidelines and international best practices. Some problems about the inappropriate exclusion and the delay in issuing the Commission criteria concerning merger control were solved by the new competition act. More independence, impartiality, transparency and accountability are guaranteed in application and enforcement of the competition law. The Trade Competition Act B.E.2560 clearly shows the development of competition law in Thailand basing on the framework of the ASEAN Guidelines.