Iran and Afghanistan, having common history and culture, had much important background for developing their relationships in the second Pahlavi Period 1941-1978. On the contrary, world powers tried to stop Iranian cultural influence in Afghanistan by making the Pashto language the official language; but the interest of the Afghan people and developing cultural relations was the main reason of preservation and the diffusion of Persian language and literature there. In spite of many upheavals in the relationships, the cultural relations was always the relying point and very reliable in developing the relations between the two countries. Considering that cultural relations are usually dependent on politics, the present essay was to study the reason of this wide level of relationships. The real interest of the two countries has caused the cultural relation to be main reason of developing relations even when there was no political relationship. This is the main hypothesis of the present essay.