The present article penned by me is on presumption of legitimacy and Muslim law. It covers a brief discussion of what is Muslim law and the various schools of the law. Then the article proceeds to discuss marriage under Muslim law where it is well settled that the nikah is a contract which has the essential of ijab and Kabul. A marriage under Muslim law can be valid (sahih), irregular (fasid) and void (batil). Irregular or invalid marriages are also called fasid. A marriage which is not valid may be either void or irregular. Void marriage is a marriage which is unlawful in itself. Then the article has been turned on the subject of legitimacy of a child. Muslim law describes that an illegitimate child is “filius nullius” which means a son of no one or a bastard. The status of legitimacy of a child is an outcome of the paternity established by existence of valid marriage. The Shia School of law says that a child who is born outside the lawful wedlock is not related to the father or the mother. It is settled law among Muslims that where paternity of a child is established there legitimacy is also established. That main pivot between legitimacy and paternity is the fact of marriage. And the fact of marriage may be established by direct proof and indirect proof .after describing the concept of acknowledgment the rules have been discussed. At the end a comparison has been drawn between evidence act and Muslim law over the law of presumption of legitimacy.