To compare Western and Islamic civilizations in terms of “family fidelity and strength” and to show the superiority of the moral foundations of Islam, the family in Greek civilization was examined and fidelity was recognized as the moral factor of family strength or continuity. This factor also laid the foundation for the strength of the family in Islam. The strength of the family is the result of the rule of moderation, morality, and the protection of the rights of the members; and the principle of fidelity, action based on the virtue of fidelity and the behaviors arising from or independent of it lead to the full commitment to the marriage contract. Despite the harmony of Greek and Islamic civilizations in relation to the effect of fidelity on the strength of the family, the moral system of Islam with a special explanation and its application in the family had a unique impact. This research expressed the role of fidelity in the strength of the family in the aforementioned civilization and Islam through library and documentary data collection and descriptive-analytical method. The results of the present study were evaluated by the Koranic verses, and the viewpoint of Islam concerning the role of fidelity in the strength of the family was also stated.