The tests on the individual cusp inclination of the posterior teeth were performed on the fifty normal subjects selected from the students of Seoul National University, Collage of Dentistry. Especially, bucco-lingual cusp inclinations and open angles were measured by means of the cusp inclination measuring instrument device by authors. The results were as follows; 1. The differences between the right and left side teeth were hardly recognized. 2. In bucco-lingual relationship, lingual cusps in upper molars and buccal cusps in lower molars revealed higher degrees than buccal cusps in upper molars and lingual cussps in lower molars 3. Generally, the differences between upper and lower teeth were not clearly defined, and the cusp angles of the molars were greater than those of the premolars. 4. The rough basis of the curve of Wilson was recognized