Under the leadership of the King Joseph II, various musical activities were flourishing in Vienna during the late 18th century. Among all, the opera buffa was one of the major forms of musical performances. During the stay in Vienna, Mozart and Theater Poet Da Ponte composed three opere buffe together, and the last one was Così fan tutte composed in 1790. Among all the opere buffe performed in Vienna from 1783 to 1791, the ratio of ensembles to arias in Così fan tutte was the highest. Therefore, the purpose of this thesis is to study the ensembles in this work, analyze the musical characterization of all characters, and carefully examine the dramatic meaning from the musical structure. In this thesis, there are three chapters except the preface and conclusion. The first chapter introduces the opere buffe performed in Vienna from 1783 to 1791 and describes its social meaning at the time, which provides the background as Mozart composed his opere buffe. The second chapter discusses the origin of the libretto and the influence of the Commedia dell'arte to the characteristic structure. This chapter therefore expresses its ideas more distinctly about two origins of the major play, the historical basis of six characters¡¦ temperaments, and Da Ponte¡¦s inductive composition technique. In addition, the third chapter further probes the musical intention of the ensembles in Così fan tutte. Furnishing characters in this play with vivid portraits and energetic vitality, Mozart composed this opera by innovative music language and various structure. Così fan tutte was harshly disparaged by critics in 19th century and being neglected for almost one century. Nevertheless, it was being highly recognized as moral standard was gradually dropped. It is fairly stated that Così fan tutte is a major achievement of opera buffa in late 18th century for Mozart¡¦s overflowing creativity in composing ensembles as well as his keen penetration for drama.