This paper investigates the symbolic meaning of ‘free verse’ especially in H. D. Thoreau’s A Week on the Concord and Merrimack Rivers. Since the early 19th Century, the Puritan tradition, New American democracy, European Romanticism and Oriental meditation philosophy like Buddhism and Hinduism have intersected or interwoven with one another. The buds of free verse began to sprout in this integral culture basis, and Thoreau followed the example of his contemporary Walt Whiman’s Leaves of Grass that has been referred to as a canon of exceptional free verse. Thoreau’s free verse is based on the eco-lyrical nature writing the concealed natural mystery or ‘unio mystica’ which has been inscribed on the surface of rivers instead of merely admiring and appreciating Nature. Thoreau regards ‘verse’ as the purest and the most ingenuous form of literature although most sentences are expressed in prosaic form as in A Week on the Concord and Merrimack Rivers, Walden, Diary etc. His proses seem to have been written in unmetrical prosody, but some contain metrical rhyme, assonance and alliteration. In this sense, this paper attempts to compare and interpret the poetic diction and lingual spectrum through the version of prose into verse or vice versa. Like this, in this version of linguistic style, we can experience the disappearing of the border line between prose and verse naturally rather than artificially. Thoreau confesses the limits of language or ‘language of silence’ in the age of noisy civilization through the paradoxical expression: “Silence cannot be interpreted in any language and even mother tongue.” Eventually, Thoreau is a inborn poet of free verse by using the oxymoron that various natural voices can be heard only through the profound and sublimated language of silence.