民國八十九年七月，政府通過施行庫藏股交易制度，並於民國九十年十一月明令禁止子公司買回母公司股票。本研究欲藉由實證研究方式探討該期間兩交易之關聯性。研究結果顯示：(1)庫藏股交易動機基本上符合股價低估假說及資本結構調整假說；子公司買回母公司股票交易動機基本上符合護盤假說。(2)買回庫藏股票公司的後續盈餘比買回前更佳；被子公司買回之母公司的後續盈餘則不比買回前更佳。再次支持兩交易買回動機分別為股價低估假說及護盤假說。(3)不論庫藏股交易或子公司買回母公司股票交易，交易期間的異常股價報酬與異常盈餘反應顯著成正向關係。表示市場投資人能正確反應公司之盈餘表現。(4)以觀察期內同時進行兩交易的公司為樣本進行研究，發現庫藏股制度對子公司買回母公司股票交易不僅無抑制作用，反而呈現同向變動。 / The listed firms in Taiwan were legally allowed to repurchase outstanding shares while the cross investment in parent company's stocks by subsidiary firms was not prohibited during the period of July 2000-November 2001. This study investigates the relationship between repurchases and cross investments in this period. 　　The findings of the study are as follows. First, the motivation of the subsidiary company's cross investments in parent company's stocks is to manipulate stock price because the unexpected quarterly earnings over the announcement quarter and the subsequent quarter are not found. However, the repurchases are motivated by stock undervaluation. Second, the empirical results show that a positive relation between abnormal returns and unexpected quarterly earnings are found in both repurchases and cross investments. This illuminates the fact that investors are able to distinguish the economic substance of repurchase from that of the cross investments. Finally, there is no substitute effect of repurchases on cross investments.