The author conducted an experiment on the relativity between the mandibular buccal and lingual cusp inclines by means of measuring full arch plaster cast teeth of the fifty student subjects of the College of Dentistry, Seoul National University. The results were as follows: 1. The statistical relativity between the mandibular buccal and lingual cusp inclines in individual tooth was hardly recognized. 2. In the cusp incline data of the whole posterior teeth, the rough basis for the harmonizing with the other occlusion factors was recognizable. 3. The cusp inclination itself was incomplete in mechanical balance, therefore, it might be said that there were a lot of possibilities that the cusp incline may act as a potential etiological factor to give rise to occlusal diseases.