The purpose of this study was to compare the retentive force of cast half round circumferential clasps, cast round circumferential clasps and wrought wire clasps on three different depths of undercut(10G,20G.30G), and to determine the proper amount of undercut for cast round circumferential clasp. Three different lower second premolar crowns were cast and each of clasp was made. Each of clasp affixed to the different types of clasp. Their retentive forces were measured with universal testing machine. The results were as follows : 1. When the depth of undercut was same, the greatest retentive force was exerted by cast half round circumferential clasp, followed by cast round - 33 - circumferential clasp and wrought wire clasp. 2. The degree of variation amount of retentive force according to depth of undercut was greatest for cast round circumferential clasp. 3. Not only elastic deformation but also permanent deformation of the retentive arm was observed after insertion and removal of the clasp. The results of this study show that the retentive force of cast round circumferential clasp falls between those of cast half round circumferential clasp and wrought wire clasp, and suggest that a 20G undercut is suitable for ensuring an appropriate degree of the retentive force.