There are four collections of Bach's chorale-based compositions for organ: the Little Organ Book, BWV 599-644; the Organ Chorale Preludes from the Leipzig Autograph, BWV 651-668; the Clavier Übung III, BWV 669-689; and the Six Chorales “von verschiedener Art”(the “Schübler-Chorales”), BWV 645-650. Among these four collections, in this article, “Schübler-Chorales” is excluded, because the organ pieces of this collection were arranged from the cantata-movements, which were written at the beginning of Leipzig Period (1724/1725). The other three collections, which were created for organ from the beginning, will be compared with one another by size, voice number and key of the pieces, and also, their decoration style of the chorale melody and the functional forms of accompaniment voices will be explored.