In the practice of clinical orthodontics, the principal goal of treatment lies not so much in the recovery of morphological and esthetic appearance of human dentition as in its functional recovery as masticatory organ. This would imply that more emphasis should be laid on the study of biting force of the mouth. Hitherto, the so-called biting force has been measured with a variety of methods by many investigators and expressed as a combined force as produced between pair of antagonizing upper and lower teeth. The value of force thus measured, however, has failed to distinguish the two important factors involved in the act of biting jaws, that is the maximum force of biting to which the antagonizing teeth can stand without aching and/or the total closing force of masticatory muscles exerting on the teeth under the control of processus mandibularis as fulcrum. In the study of biting force, therefore, it is essential to distinguish these two factors and measure them respectively for their relative magnitude. Present study was undertaken to measure the maximum force which individual tooth can tolerate in biting and the total closing force of muscles, because no distinction was so far made between the two. The total closing force of muscles was also studied with relation to ages of subjects examined. Results were summarized as follows : 1. Significant differences were shown to exist between the biting force as measured on the dental arch and the one measured at the middle of sagittal plane, so that one would not be able to predict anything about the difference of biting force possibly existing between right and left dental arches on the basis of data from measurement at the middle of sagittal plane. This would lead to the conclusion that measurement of biting force as needed in the orthodontic practice should be conducted on the dental arches. 2. The biting force of masticatory muscles as exerted on those part of dental arch occupied by the premolar and molar teeth was essentially a kind of momental leverage action with processus mandibularis as its fulcrum. 3. When the total closing force exerted on individual tooth and the maximum force of tolerance of each tooth was compared, it was shown that the latter force measured in the anterior teeth was considerably smaller than the former, while the same force measured in the molar region was approximately equal to the total closing force. 4. In those subjects who showed greater closing force also showed greater value of tolerance of their teeth. 5. The closing force tended to increase with advancing age in subjects from 9 to 14 years of age and in the male subjects the value of the closing force was higher than that in the female. 6. When the closing forces of right and left sides of dental arch were measured respectively on two groups of subjects whose habitual side of chewing was either right only or both sides by subjective judgement, the data obtained agreed with the subjective judgement in more than half of the subjects examined. However, the data obtained from those subjects who claimed that their habitual side of chewing was left side of jaw did agree to the extent of only one third of the total.