It was proven that the letter format of the Prophet delivered to the kings during his da’wah was astounding. His letters being written by Zaid bin Thabit were always preceded by Basmalah word, whereas the letters were sent to unbelievers. This study is a kind of descriptive qualitative research that used a historical approach. The result shows that there were some reasons behind the Prophet messages as a medium of da’wah to the kings: a) it was a result of the Hudaybiya Peace Treaty and the success of the Prophet on establishing Muslims force in Medina. It was influenced by the conflict between King Heraclius and King Kisra Abrawaiz. b) the Prophet’s letters were likely to show that the Prophet Muhammad was the Medina leader. c) it was trig...