This paper mainly discusses the changes happened between the US-Soviet relations before and after the implementation of the Lend-Lease Act, with an intention to illustrate the impact on this sort of international relations by virtue of the Act. With the Act as their joint, the United States and Soviet Union, the main members of the Anti-Fascist Camp, temporarily set aside their ideological divergence and established military alliance. The vast amount of military materials, provided by the United States through the Lend-Lease Act, have played a positive role in relieving the adverse conditions faced by Soviet Union at the beginning of the war. The relations between the two countries, however, deteriorated as the war came to an end. Shortly after the triumph over Fascist forces, the U.S. abruptly terminated lend-lease supplies for Soviet, the distrust of America and other west countries increased in Soviet, and the weak alliance thus fell apart.