The temporary effects of muscle atrophy, followed by chemodenervation due to acetylcholine blockade at the neuromuscular junction by botulinum toxin type A(BTX-A) were mainly used for the treatment of hypertrophied muscle. In the treatment of masseter hypertrophy or cosmetic contouring of the lower face with injection of botulinum toxin type A, these muscle atrophy causes the change of the bite-force inevitably. Although several reports about the histological change of muscle, ultrasound and computerized tomography measurement studies of the change of muscle thickness in the treatment of masseter hypertrophy with injection of BTX-A have become available recently, it is necessary to study further the change of the bite-force and the relationship between the bite-force and clinical effects after the botulinum toxin A injection. The aim of this study was to evaluate the change of maximal bite-forces after botulinum toxin type A injections for treating the contouring of the lower face. Botulinum toxin type A(BTXA(R), Lanzhou, China) of 25U per side was carried out in 7 subjects. The change of maximal bite-forces was evaluated at pre-injection and 2 week, 4 week, 8 week, 12 week post-injections using bite- force measuring machine(MPM-3000, Japan). There were statistically significant differences between pre-injection and 2 week, 4 week, 8 week post-injections(p<0.05). However, there was no significant difference between pre-injection and 12 week post-injection. In conclusion, the change of maximal bite-forces after BTX-A injections according to the period was similar to the histological change of the muscle. The clinical effects last more longer than maximal bite force. Therefore, it is necessary to evaluate further these difference with more larger samples and the relationship between the change of maximal bite force and clinical effects.