On April 2, 2007, The United States Supreme Court, by a vote of 5-4,rendered its decision in Massachusetts v. EPA, and remanded the case for further proceedings. Massachusetts v. EPA contains four landmark rulings. First, the Court held that Massachusetts has standing to petition for review of the EPA's denial of their rulemaking petition. Second, the Court held that carbon dioxide and other greenhouse gases are "air pollutants" under the Clean Air Act. Third, the Court held that the EPA is authorized under section 202(a)(1) of the Clean Air Act to regulate green house gas emissions from new motor vehicles once the EPA forms a "judgment" that greenhouse gas emissions from new motor vehicles contribute to climate change. Finally, the Court held that the EPA may only avoid taking regulatory action under section 202(a)(1) with respect to greenhouse gas emissions from new motor vehicles when "it determines that [1]greenhouse gasses do not contribute to climate change or [2] if it provides some reasonable explanation as to why it cannot or will not exercise its discretion to determine whether they contribute to climate change."This case is considered as one of the most important cases in U.S. environmental law in the 21st century. This paper tries to closely examine the fundamental reason why the Supreme Court made a 5-4 decision in this case, and concludes that it is attributable to the difference of opinion of the Justices in the Supreme Court on the legal principle of American law. They are classified as Conservatives and Liberal. Briefly speaking, in the interpretation of the Constitution and federal law, Conservatives put much importance on the protection of liberty and right of the people, recognizing the limitation of liberty and right for public interest at the minimum. Liberal group is intended to recognize the limitation of liberty and right for the purpose of the public interest in a wide range. In the area of environment law,Conservatives assert to solve with common law principle when the damage occurs due to pollution, recognizing standing at the minimum. In contrast,Liberal group intends to focus on the preventive regulation on the environment pollution, recognizing standing as large as possible.