Most research on the history of American women's political participation failed to explore in-depth the process that women learned politics and became politically active. Historian Nancy F. Cott suggested that after 1920, American women had used female voluntary associations as vehicle to extend their participation in civic life. The League of Women Voters of the United States, founded in 1920, aimed to educate and train American women to become effective voters and responsible citizens.By tracing the League's values, philosophy, and content of civic actions, I analyze how it had enhanced women's political participation. I found that four characteristics of the League enabled it to attract women to join and to receive political education and training. Four characteristics are as follows. (1) Core principles: democracy and feminism; (2) Extended from women's rights to human rights and public interests; (3) Work philosophy: nonpartisanship, consensus on issues, study before action, and concerted advocacy; (4) Civic actions consisted of civic education and civic advocacy.