In 2018, the Presidents of Azerbaijan, Iran, Kazakhstan, Russia and Turkmenistan signed the Convention on the Legal Status of the Caspian Sea (the Aktau Convention). Despite this, some specific issues related to the straight baseline, remain unresolved. The result is that Iran has yet to ratify the Convention subject to negotiations concerning a separate agreement in the future. Since 2018, the signatories to the Aktau Convention have not reached an agreement in the course of negotiations on a separate agreement. The main question raised in the article is why Iran ha s not ratified the Aktau Convention yet. The paper shows that the authorization of the convention under Para. 4 Art. 1, the specific shape of Iran’s coastline, and the lack of agreement in the negotiations related to the special agreement prevented Iran from ratifying the Convention. The paper finally concludes that the border delimitation considered by Iran differs from the border delimitation agreed on by the four other states, namely Russia, Kazakhstan, Azerbaijan and Turkmenistan, and it is only when the straight baseline is established with respect to Iran’s will and model that Iran will complete the ratification process.