This study was conducted to clarify functional differentiation among the anterior temporal muscle (AT), the middle temporal muscle (MT), and the posterior temporal muscle (PT) for producing bite force with various direction and magnitude. Six male subjects were asked to bite arbitrarily and the direction and magnitude were recorded using a three-dimensional bite-force-transducer. Concurrently, electromyogram of muscles was done. As a result, each muscle had a proper range of bite force direction with its range wider in AT and narrower in PT. Each muscle had a specific direction shown by highest EMG activity, anteriorly in AT and posteriorly in PT. In AT, the activity was higher than the others with anterior bite force direction. In MT, it was higher than the other two when the direction of bite force was posterior. In PT, the proportional activity to the combined muscle activity increased as the bite force direction changed to posterior, with an opposite tendency in AT. Changing the direction of bite force laterally had no effect on the proportion of muscle activity. The findings indicated a functional partitioning of the temporal muscle into at least three parts. The direction of muscle fibers seems to dictate the characteristics of muscle function.