The United States Freedom of Navigation program is one of the diplomatic and military projects initiated in 1979 by Jimmy Carter Administration and has been implemented, continued, refined, and improved by American governments ever since. The paper first defines the US FON program and briefs its history. It then delineates the U.S. practice of the program in the China Sea, discussing its characteristics and changes in different presidential terms. Suggestions of the countermeasures China might take to meet the challenges are presented afterwards.