Developments in Syria have been one of the most important events that have influenced the foreign policy and consequently, national security of Islamic Republic of Iran in a way that Syria’s condition has become a game with numbers for Iran and any fundamental development in its political structure significantly influence Iran’s security. For years, Syria has had the role of a gate for Iran to Arabic countries as well as resistance axis and strategic links to Mediterranean countries and the Near East. This caused that Syrian weakness and overthrow of Bashar al-Assad led to the weakening of the resistance axis and reducing Iran’s regional influence. The present study deals with the following aims:

1. The explanation of the strategic importance of Syria for Islamic Republic of Iran,
2.  Explaining the type of Syria’s development influence on Iran’s national security in different fields,
3. Explaining the type of Iran’s competition with regional and trans-regional powers, and
4. Explanation of Iran's security strategy against developments in Syria.

This study also examines the hypothesis that in the case of continuation of the crisis in Syria, Iran’s national security will be decreased due to increasing threats from national and transnational rivals. To evaluate the hypothesis, the author has used “balance of threat” theory by which threats facing Iran in Syria can be analyzed. The findings show that crisis in Syria is a threat for Iran’s security and because of the unbalanced threats against Iran, Iran attempts to solve the crisis to reach the balance of powers.