BACKGROUND Flexible flatfoot is associated with altered plantar pressure distribution, but it is not clear how muscle fatigue affects plantar pressure characteristics in flexible flatfoot and normal foot. OBJECTIVE To investigate the effects of calf muscles fatigue on plantar pressure variables in flexible flatfoot and normal foot. METHODS Twenty-five people with flexible flatfoot and twenty-five people with normal foot were included. The unilateral heel-rise test was used to induce calf muscles fatigue. Plantar pressure variables were collected during preferred walking immediately before and after fatigue. The two-way mixed-design ANOVA was used to determine the main effect of fatigue and the interaction between foot posture and fatigue. RESULTS Fatigue caused medialization of the contact area under the forefoot and the maximum force under the heel and forefoot (p< 0.05). When examining the differences in the effects of fatigue between groups, the contact area under the medial heel increased with fatigue in flexible flatfoot but decreased in normal foot; moreover, the contact area and maximum force under the midfoot and the maximum force under the third metatarsal decreased with fatigue in flexible flatfoot but increased in normal foot (p< 0.05). CONCLUSIONS Calf muscles fatigue caused medialization of the maximum force and contact area. Especially the midfoot was affected differently by fatigue in flexible flatfoot and normal foot.