Objective: The objective of this in vitro study was to determine the peak brushing force generated by toothbrushes of adult and children with different bristle stiffness. Methods: A total of 33 toothbrushes were used in these experiments. The toothbrushes were divided into 3 groups, adult with soft bristle, adult with medium bristle and children. The peak force generated by the toothbrushes were recorded by fixing the handle of the toothbrush with the force transducer (UFI, CA,USA) with a wire. The transducer was connected with a Physiological Recording System (Biopac®, MP 100A-CE, Biopac System Inc. USA). The bristle tips of the toothbrush were pressed on the teeth in the dentoform. Then, the peak force generated by the toothbrushes were recorded with the system into the computer and analyzed with AcqKnowledge programme (version 3.5.7, USA). Results: The means peak brushing force (±SD) generated by the adult toothbrushes with soft bristle, adult toothbrushes with medium bristle and children toothbrushes were 301.22±53.04, 283.17±40.31, 209.18±62.09g, respectively. The peak force generated by the children toothbrushes were significantly lower than the adult toothbrushes (p<0.05, 1 way ANOVA, Tukey test). There were no significant different between the medium and soft adult toothbrushes. Conclusions: It was concluded that the children toothbrushes generated significantly lower peak brushing force than the adult toothbrushes. Besides bristle stiffness, handle design also has an effect on the force generated by the toothbrushes.