國際私法統一協會於1994年通過並於2004年擴編的國際商事契約通則雖非國際法意義的國際協定,但不僅對世界許多國家契約法之制定與修正影響甚大,在國際商事紛爭之解決上,也扮演愈來愈重要的角色。本文認為契約法之法源與單純因法律規定而生之法律關係不同,當事人之約定乃最重要之法源,且在複雜的商事契約,契約解釋乃至於契約補充解釋之重要性,遠逾內國立法者所制定之任意規定。從而,聯合國商事契約通則若經當事人援用,當能成為約定之內容。即使無法如此,聯合國商事契約通則規定中,若可認為乃契約一般法律原則之明文,則未始不可以成為法律發現法源。
Although the second edition of Unidroit Principles of International Commercial Contracts (PICC) is not a legally binding instrument, it does not remain a dead letter but is actually applied in practice. This article tries to show that the most important sources of the law of contract are not the regulation made by the national legislator, but the terms expressed or implied in contract. Just like other soft law instruments, PICC may be applied when the parties agreed that their contract be governed by them. By virtue of the persuasive value of the PICC, they may be used to interpret or supplement international uniform law instruments and domestic law.