In this article, I have reviewed the definition of international mercantile customs, their preferential application, the developing status of the credit practice, the effectiveness and relationship of the international standard banking practices, e.g. UCP 600 and ISBP 2007, ISP98, URR 725, eUCP 1.1. and the like, established by the International Chamber of Commerce (ICC). It is important to emphasize that the autonomous agreements between the credit parties and the international practice on the credit transaction are respected above all because of the special nature of its transaction. When we want to apply to a letter of credit by the international rules - UCP 600, ISP98, URDG, URR 725 and eUCP 1.1, we must indicate expressly in the text of the credit that it is subject to the respective rule. But the International Standard Banking Practice, 2007 revised by the ICC is applicable to without its indication in case of the UCP 600 credit. On the other hand, the UN Convention on Guarantees and Standby Credits applies to an international undertaking referred to in its article 2, (a) if the guarantor/issuer is in a Contracting State, or (b) if the rules of private international law lead to the application of the law of a Contracting State, unless the undertaking excludes the application of the Convention. And this Convention applies also to an international credit not falling within its article 2, if it expressly states that it is subject to this Convention.