This paper reviews the judicial evolution of public interest of land expropriation in the United States, and uses the economic theory to analyze the efficiency implication of public interest. The Supreme Court of the United States has gradually broadened its interpretation of the “public use” of the Takings Clause in the Fifth Amendment, from a narrower, literal view to a much broader one. In the end, “public purpose” is equated with “public use”. However, it's difficult for the court to draw a clear line between “public purpose private expropriation” and “pure private expropriation”. So, people become worried about the lifting of the floodgate of abuse of land expropriation and overencroachment of public rights on private rights. Land expropriation can improve allocation efficiency of land, and may eventually compensate the loss of those being expropriated and improve their welfare through the acceleration of development of economy.