Objective: The purpose of this study was to compare ankle function between adults with and without Down syndrome (DS). Design: Cross-sectional study. Methods: Ten adults with DS and 18 without participated in this study and underwent manual muscle test (MMT), range of motion (ROM) assessment, star excursion balance test (SEBT), and functional movement screen (FMS). The tests were demonstrated to increase their accuracy and the actual measurements were assessed after one or two demonstrations. To minimize the standby time and fatigue, the travelled distance and measuring order were adjusted. To remove the influence of shoes on the measurements, the shoes were taken off and only socks were worn. Results: Dorsal and plantar flexion MMTs of both ankles were significantly weaker and plantar flexion ROM of both ankles were significantly lower in adults with DS compared with those without (p<0.05). However, dorsal flexion ROM of both ankles were not significantly different between them. There were significant differences in distances measured in all the directions (anterior, anterolateral, lateral, posterolateral, posterior, posteromedial, medial, and anteromedial directions) of SEBT (p<0.05). Significant differences were also demonstrated in the scores of hurdle step, inline lunge, shoulder mobility, and rotary stability among the seven items of FMS (p<0.05). Conclusions: To enhance the dynamic stability of adults with DS, it is necessary to improve ankle stability by strengthening the ankle dorsal and plantar flexors.