Single epithet is a type of epithets which follows its own description in case of the expression, ‘ known’ and ‘ unknown’ , gender, and number. It is used for different ‘ purposes’ including ‘ allocation’ , ‘ clarification’ , praise, admiration, blame and criticism, emphasis, ambiguity, ‘ detail’ , ‘ extension’ , and pity. This research investigates the single epithet along with its purposes and justifications in the letters of Amir al-Mu’ minin Ali (pbuh) in the Nahj al-Balagha, using the descriptive analysis method. The findings of the research show that the two justifications of ‘ allocation’ (i. e. the restriction of shared circle of the ‘ unknown’ ) and ‘ clarification’ (i. e. the removal of the ‘ known’ possibilities) have high frequency in the letters of the Nahj al-Balagha. Also the ‘ purpose’ of praise and admiration is first addressed to God Almighty. After the essence of Almighty, it is addressed to the Holy Prophet, the Family of Prophet, and the Righteous, respectively. Moreover, the adjectives of blame and criticism are addressed to Satan and ‘ Passion’ and the ‘ purposes’ of emphasis and ambiguity are seen in the Imam’ s speech with less function.