In 1998, als the first time of official declaration, all ten governments in Southeast Asia agreed to try and rid the region of illicit narcotics and towards 'drug-free zone' in 2020. Four year later, in April 2012, the Association of Southeast Asian Nations (ASEAN), in collaboration with the United Nations Office on Drugs and Crime (UNODC) center for East Asia and the Pacific, reaffirmed the Joint Declaration for a Drug-Free ASEAN by 2015, an ambitious plan which focuses on increasing interdiction and eradication operations, along with use prevention, in an effort to remove illicit drugs from the region.Throughout the past two decades, the UNODC has monitored the evolution of drug trafficking in Southeast Asia to assess the feasibility of achieving a drug-free ASEAN.Although national and international authorities failed to understand the dynamics of drug trafficking in the Golden Triangle, the extensive efforts of law enforcement agencies of ASEAN have been continuing to control the rise of new producing areas, trafficking routes and even substances. This paper provides an overall situation of drug trafficking in the Southeast Asian region and its latest trends before reviewing current legislation of ASEAN to prevent and combat drug-related crimes. Besides, the cooperative mechanism to support authorities of ten members in fighting drug trafficking will also be discussed and analyzed. Finally, some practical recommendations to improve the effectiveness of regional cooperation to combat the flows of illicit drugs in the region that also call for further researches and exchanges.