The U.S. Department of Justice has been investigated allegations that IBM abused its dominance of the mainframe computer market to harm rivals by refusal to license for its operating systems to customers who use non-IBM hardware, retaliating against business partners deemed disloyal and bundling its mainframe operating systems with hardware. It involved a selective refusal to license its patented technology to the software developers (FSI and PSI), which was an essential factor in developing a platform supporting IBM’s mainframe operating system. As a result, the competition in the mainframe platform market was harmed, and thereby denies consumers the freedom to choose. Although the U.S. District Court dismissed T3’s claims against IBM, the DOJ and the Second Circuit has reviewed the case again as the decision was open to criticism. This is a significant opportunity to draw a clearer line whether an unilateral refusal to deal in a lawfully patented product violates an antitrust law since Microsoft case of a decade ago. In September of 2011, IBM offered a concession to European regulators, including a commitment to make spare parts and technical information easier for competitors to provide maintenance services for its mainframe computer. As the EU decided to close an antitrust probe against refusal to deal, the unlawfully tying its mainframe hardware with its operating system is now under question. This article examines the IBM decision more critically, and emphasizes that the selective or discriminatory refusal to license patented technology, which is an essential element to competitors in adjacent market, could bring a more anti-competitive effects than other types of refusals. On this point, the European approach and standard is more reasonable and clearer for decide the legality of an unilateral refusal to license intellectual property. It also addresses some of issues on compulsory license as a remedy for refusal behavior.