Although gift causa mortis has been deemed as the same as devise concerning their gratuitous, obligatory, property-related, and mortis causa nature, and their social economic function, devise shall take the form of a will and does not require counter parties as opposed to gift causa mortis which needs mutual agreement as a contract between both parties for the effectiveness. In recent years, lots of cases, in which the devise has been invalid due to defects in the formality of a will, have occurred, and followed by one issue, that is, whether such invalid devise may be converted into gift causa mortis. Judicial practices essentially take negative position towards this question. By analyzing the meaning, nature and differences of both, it can be recognized that generally, when a devise is invalid due to defects in the formality of a will, the intention which the testator is giving the gift to the devisee can be legally presumed, unless the testator stated otherwise. In this situation, such intention may be converted into an offer to conclude a contract of gift causa mortis. However, such contract shall not become effective until the devisee accepts. Whether a devisee has accepted the gift or not shall be determined based on the circumstances of each case, not by a general rule.