The organ, more than any other musical instrument, has assimilated numerous changes and experiments. A number of distinct national styles evolved throughout the history of organ music, spanning almost 2000 years. The classical French organ music, roughly between about 1650 and 1790, and the French organs of that period have developed a unique relationship. The musical textures were perfectly molded to the indigenous characteristics of specified combinations of stops. By 1650, French organ composers had accepted the new musical style which established registration practices for specific musical types on the organs whose specifications had become rather standardized throughout France. Organ music of this period in France became confined to forms which were used by all French organ composers of this period. Registration indications became titles for the pieces. It is fourtunate that many composers of this period wrote detailed instructions, perhaps the most thoroughgoing of any epoch, in the prefaces which they included in their collections of organ music. These instructions provide us with a valuable informations on the interpretation of their music. The organs of our country bear practically no resemblance to the French classical instruments. Therefore, the French organ music was seldom performed and studied. The purpose of this study is to understand the language of this particular style of organ and the performance practice of their music, apply this information to present day organs of our country and help the performers to express the composers' intentions fully in their performances. This study is consisted of observations on French classical organ building, organ music literature, performance practice(registration, Notes Ine´gales, articulation and ornamentation) and finally, an analysis of Francois Couperin's Messe a｀ l'usage des Convents, a typical work of the French classical organ music.