Talking about the importance of names, particularly God’s names, is a complicated issue. In ancient era the name of a person or a deity was equal to his/her identity, therefore, knowledge of names had a magical function. The subject of name has been markedly expressed in Old Testament. God has revealed there by several names, such as El, El Shaddai, Elohim, Adonai and YHWH. Some of these names are very ancient and others are of Semitic origin. El is a common term in all Semitic tribes and YHWH is the personal name of the God of Israel. Calling the personal name of God was taboo and therefore considered as a great sin. So for expressing reverence the sacred name was replaced by the term Adonai (Lord). Every name in Old Testament reveals different aspect of the God of Israel, for example, El is a compassionate God, and Elohim is the Lord of Judgment. In some passages YHWH is the God of mercy, while in others is portrayed as an avenger. This essay, with a descriptive-historical approach, tries to shed light on this issue.