Beethoven’s 32 piano sonatas have remained staple in piano repertoires since each of them features unique styles and characteristics uncommonly observed in Haydn and Mozart. The differences between numbers are extreme, not only just between Opus number but also within opus number. The impact of the 32 sonatas upon romanticism was immediate and they quickly challenged composers to redefine the standard writing of sonata. One of the most intriguing sonatas is both numbers of Op. 27, entitled “sonata quasi una fantasia,” in which there had not been such inscription in any previous keyboard sonatas since its origin in the late Renaissance era; nevertheless, Beethoven did not express adherence to the fantasia writing expected in Renaissance/Baroque style; therefore, the term “fantasia” becomes interesting to evaluate as Beethoven implies a new “fantasia” concept which made him a composer of his own class and became immediate impact to the romantic style and its composers. Qualitative study is used for this research, along with analytical study on important elements to write the conclusion, including the melodic/textural writing, the harmonic construction, and the structure of the music.