This paper is to analyze how Walt Whitman developed the theme and structure of Leaves of Grass with his ideal of democratic vistas. Whitman established his identity as an inspired poet, having faith in the divinity of man based on transcendental belief. After being awakened to the transcendental truth, he practiced his own common world view--his democratic vistas. Whitman searched for the unity with nature and identified his self with "common man and his nation." The poetry expresses "cosmological and national ideology" dedicated to the creation of an ideal nation united in eternal freedom and peace. By portraying common cosmic and national theme in terms of his individual personality, he brought various paradoxical and controversial ideas into one thing, namely "democracy", fusing diversity into unity. As in the symbol of the grass, there is a unity in variety reflected by democracy in a cosmological and political compound. With the form of free verse, he could express his liberal unrestrained and mystical thoughts of democracy. This new form has been associated with the poet's strong consciousness of the need for modernization in his country. He willingly assumed "the role of prophet and public voice for American democrat" with the rolling catalogues and I-persona which formed a sense of the common man and common things of America. Whitman pioneered a democrat literature with simple and dynamic tone and style. He successively pursued the democratic vistas in his Leaves of Grass.