我國民法第507條第1項揭示了承攬契約中定作人的協力義務，同條第2項明文規定承攬人於解除契約後，並得請求賠償因契約解除而生之損害。學說上對此義務之性質看法分歧：有認為係定作人之真正義務者，亦有認為係不真正義務者。又不論是採真正義務說或是不真正義務說之論者，對定作人違反協力義務之法律效果亦各持不同看法，呈現百花齊放的景況。本文透過過往學說的爬梳，以及近年來牽涉定作人違反協力義務之多起判決的簡要分析，從「理論」與「實際」二層面解析定作人協力義務之性質以及違反效果。本文認為，既然定作人違反協力義務常造成承攬人固有財產利益之損害，應將其定性為真正義務。定作人之協力義務與報酬之提出無關，乃是為了避免承攬人於因定作人違反協力義務而增加費用支出，故協力義務在契約義務群中之定位，應為附隨義務，或稱保護義務。在違反協力義務之效果部分，現行民法第507條第2項應改為承攬人得「終止」契約，以平衡定作人與承攬人之利益。

Article 507 paragraph 1 of the Civil Code reveals the proprietor's duty to cooperate in the contract. Paragraph 2 of the same article provides that the undertaker may claim compensation for damages arising from the rescission of the contract. There are different theories on the nature of this obligation: some consider it a genuine obligation of the proprietor, while others argue that it is not a genuine obligation. In addition to the debate regarding the nature of this obligation, there are also different views on the legal effect when the proprietor fails to cooperate. This article analyzes the nature of the proprietor's duty to cooperate and the legal effect that occurs when a proprietor fails to fulfill this duty from both the "theoretical" and "practical" perspectives. Through reviewing past researches and a brief analysis of recent judicial decisions concerning the breach of proprietor's duty to cooperate, this article argues that since a breach of duty to cooperate by a proprietor often causes damage to the inherent property interests of the undertaker, this duty should be characterized as a genuine duty. Furthermore, while the proprietor's obligation to cooperate is not related to the offer of remuneration, its purpose is to prevent the undertaker from spending additional expenses caused by the breach of the proprietor's obligation to cooperate. Therefore, the nature of the proprietor's duty to cooperate should be positioned as a sort of "collateral obligations", also known as "protective obligations", in the groups of contractual obligations. As to the legal effect of the breach of proprietor's duty to cooperate, Article 507 paragraph 2 of the current Civil Code should be amended and should allow the undertaker to "terminate" the contract instead of "rescind" the contract in order to balance the interests of the proprietors and the undertakers.