As one of the most popular means of trade in the world, shipping by sea has always been subject to maritime hazards. Thus, the implicit commitment of a ship lease contract is that the transport operator provides a seaworthy ship. There is, however, no specific definition of the legal nature of this obligation in most conventions or international laws, and it is only in this regard that the statement of seaworthiness is cited as an implied obligation. Having been written in the descriptive-analytical method, this study attempts to explain the legal nature of this obligation, its position among absolutes or relatives, primaries or subsidiaries, implicit fundamentals or customs, its compliance with the conditions of article 234, the burden of proving seaworthiness, and the lack of performance guarantees caused by its absence. According to the results of the study, a transport operator is under a relative obligation to provide a seaworthy vessel. The existence of this obligation can be mentioned both as a main condition and a secondary condition, and if there is no specification in the contract, it is referred to as a customary implied obligation. Additionally, the condition of seaworthiness would be close to the verb condition in accordance with Article 234 of the Civil Code of Iran. For a claimant (the owner of the goods) to prove a lack of seaworthiness, they only need to provide evidence that the loss has occurred. For the sea transport operator to be relieved of responsibility, s/he must prove that s/he took the necessary precautions at the start of the voyage. The owner, otherwise, is responsible for compensating the victim for the damages caused by the violation of unseaworthiness by restoring the previous situation by providing the property and if an excuse is provided, by supplying a substitute.