This paper deals with contract modification in international sale of goods, where the parties are free to conduct any modifications by the mere agreement without following any formal requirements. However, the Vienna Convention on Contracts for the International Sale of Goods (The CISG) recognizes the need for contracting parties to introduce a certain degree of legal certainty into their relationship, which is directly facilitated by the legal framework of the no-oral modification clause provided by the Convention. No-oral modification clauses represent contract provisions stipulating that all amendments to the contract must be in writing or in some other specified form. Therefore, the paper analyses the nature and legal character of such clauses and examines the ways in which they should be interpreted to determine the specific intent of the parties when incorporating them. Additionally, in practice, it is common for contracting parties to subsequently act as if such clauses do not exist in their contract - fulfilling their contractual obligations in accordance with the informally amended provisions of the contract. In such cases, the Convention prescribes a preclusion from invoking the clause as a protection against contradictory behaviour in accordance with the estoppel principle. It is important to outline the prerequisites for the application of this provision while also emphasizing its limited effect. The paper also discusses the possibilities for informal modification of the agreed formal requirements for amendments.