Caspian Sea had been used a long time between Iran and the Soviet Union based on agreements of 1920 and 1940, without international importance. After the Soviet’s collapse and with increasing the countries of sea border from two to five, this lake was important due to geopolitical and geo-economical conditions. Following these situations, the legal regime of Caspian Sea needed to be revised. With paying attention to existing challenges among borders countries for accessing justice legal regime, the Islamic Republic of Iran could not have accessed to appropriate legal regime. This paper is looking for just legal regime of Caspian Sea and the reasons that Iran has not accessed to its appropriate share by paying attention to structural – agent approach.