Many studies have been accomplished on the subject of Cyrus Cylinder inscription, but mythological role of Mesopotamian Nabu on this cylinder has not been properly studied. The Cylinder is damaged on some parts and Nabu and Marduk are the only remained god-names in the inscription. Nabu was the god of writing, wisdom, and to some extent happiness who protected scribes and was believed to be the Marduk’s minister or son. The authors’ aim, in this article is to understand why Cyrus had Nabu be mentioned beside Marduk in his famous cylinder which was ordered to be carved on his peaceful triumph over Babylonia, and why no other god-name was inscribed there. Therefore, Nabu’s function, temple and origin are initially studied and then his different roles in kings’ inscriptions in different times and places, and also the importance of attendance of Marduk and Nabu beside each other are studied. Finally, Nabu will be analyzed in Cyrus Cylinder and it will be made clear, as the result, that Nabu had probably an important role in the said inscription as Marduk’s son and heir, and the one who empowered and protected kingdom of Cyrus and his successor, Cambyses, relying on wisdom, longevity and happiness.