In these days, transnational illicit trafficking cases in narcotic drugs like methamphetamine, marijuana as well as smuggling cases of buying illicit drugs directly from a foreign seller on the internet are increasing sharply. Customs inspection and controlled delivery are recognized to be one of the most effective tools to combat transnational drugs trafficking cases by an international mail. The 2013do7728 ruling of the Supreme Court of Korea has a meaning that it is the first ruling about the legal character of Customs inspection of an international mail and the legality of controlled delivery. In this case, the Supreme Court of Korea decided that customs inspection of an international mail without a search and seizure warrant is legal because considering the purpose of Customs Act and the procedures provided in the Act, notifications and instructions of Korea customs service, customs inspection is not a legal disposition in the criminal procedure but an administrative investigation. Controlled delivery enables the law enforcement agencies to specifically identity, arrest and prosecute not only the carriers and couriers but also the principals, organizers and financiers of such illicit activities. Under the controlled delivery system, the delivery of an international mail such as narcotic drugs is monitored closely by law enforcement officers in order to identify as many members of a trafficking network as possible and to arrest them at a point where legal proof is most readily available. And the Court decided that controlled delivery is not a seizure as a legal disposition of taking possession of a property and after the delivery, the seizure of narcotic drugs by the investigative authority with a consent of the accused is also legal.