This study is to vindicate the vacation of arbitral awards in the United States. It focuses on the harmony of case law with statutory law of the United States. Until the early twentieth century, the American legal system, having adopted the English common law view, harbored a hostile attitude toward arbitration. The purpose of the Federal Arbitration Act (FAA) of the United States, enacted in 1925, was to eliminate the hostile attitude of courts toward arbitration. Congress is to enforce arbitration agreements into which parties have entered and to place arbitration agreements upon the same footing as other contracts. The structure of grounds for vacating arbitration awards has two layers. One is of vacating grounds with statutory origins, such as the FAA and the Uniform Arbitration Act, and the other, of vacating grounds originating from a nonstatutory, case law background. For a while, vacatur based on case law has coexisted with vacatur on statutory grounds for arbitration awards. After the Supreme Court decision in Hall Street Associates, L.L.C. v. Mattel, Inc., however, the justification of vacating based on case law has weakened. Post-Hall Street decisions of circuit courts show ways to deal with manifest disregard of the law. One of them is the harmonization of the case law grounds for vacating with the statutory grounds. It seems that the manifest-disregard-of-law and public-policy exceptions show a possibility of survival after Hall Street. However, other nonstatutory grounds for vacation of arbitration awards have no firm basis after Hall Street.