本文從因事而祭的角度，探討包山楚簡禱祠祖先的祭品及其異同原因。首先，觀察東周傳世文獻中因事祭祖的祭品內容，以作為論述楚簡禱祠祖先的背景。其次，包山楚簡禱祠祖先的時間約於西元前318-316 年之間，這三年間祭主昭佗皆任左尹，大夫身分不變，因而轉從受祭者身分與關係著眼觀察用物異同情形，得知祭品兼具共相與參差情形。其三，探討禱祠祭品異同原因，包括祭品因祭主、受祭者、舉禱動機、禮儀種類等整體情境而具有相對性，不同的祭法將影響祭品的種類，祭品與祭法因傳統的權威與規範而帶有固定性，祭品因卜筮程序與禱祠方案者與鬼神的約定等因素而產生變動。值得注意的是，禱祠中的卜筮程序將使祭品的出現富含或然性，因而即使用物現象相同卻不全然能以規則視之，相同的現象不代表本質也相同。上述研究將有助於補充傳世文獻對禱祠記載的不足，提供禮學或楚簡研究者參考。

From the viewpoint of sacrifice by motives, this paper aims to discuss the usage of offerings and inspect the reasons of variation. First, it explains that dao（禱）is the worship with specific aim for the matter. Based on the information, dao（禱）can offer different sacrifices to ancestors regularly or irregularly. Second, it inspects the usage of offerings. Some are rules and some are not. Third, it inspects the reasons of variation. The offerings are different from the master, ancestors, motives, and the classification of rite. It is worth noting that divination will make offerings with probability, than the same usage of offerings cannot be regard as rules. In all, this paper boosts the understanding of the worship bamboos in Chu and may give reference to scholars.