Numerous provisions are connected to the notion of "the fundamental breach of the contract" in the International Convention on the International Sale of Goods (ICISG) among which are the right to cancel of the seller and buyer, right of the buyer to request the substitute goods, transferring the transactional guarantee and the predictable breach. The aforementioned concept is one of the most important and challenging concepts in the said Convention. Other international documents have used "fundamental non-performance of the contract" instead of that concept. The present article aims to answer this question that whether "fundamental breach of the contract" and "fundamental non-performance of the contract" are a single concept or are two distinct concepts, and if they are two different concepts, what are their similarities and distinctions? In order to answer this question, these two concepts are studied and analyzed in terms of definition, elements and effects. In the end, we will examine whether these concepts or similar cases are mentioned in Iranian civil law, and if not, to what extent are Iranian legal entities closely associated with these concepts? The Through this study it is determined that despite the difference in the definition, elements and effects of these two concepts, there is no substantive and basic difference between them. Furthermore, however the concept of a fundamental breach is not seen in the Iranian civil law, the mechanism embedded in the law of Iran (options) is close to the mechanism of the ICISG. In addition, the external instances of termination under Iranian civil law are far less than the external instances of termination under International Sale Convention.