The structure and function of the flatfoot are different from those of the normal foot. To identify their differences, the rearfoot motion, the arch index, and the lateral arch angle are important reference points. This study was examines the relationship among these points. A total of 230 service men were measured for their arch indexes and arch angles using an X-ray and a footprint pedal. Another 25 flatfooted (169.9± 3.8cm, 63.6± 10.9kg) and normal footed men (171.6± 4.9cm, 62.5± 7.4kg) were measured for their rearfoot angles using a Peak Performance System (120 films/sec) in static standing, walking (4.8km/hr), and jogging (9.6km/hr) conditions. The one-way ANOVA was used to analyze all collected data (α=.05). The results are listed below: 1) There was a positive relationship between the arch index and the arch angle, with the related coefficient at 0.36.2) In static standing, walking and jogging, the pronation angles and the range of motion (ROM) of the flatfoot were significantly larger than those of the normal foot. 3) Both ROMs of the flatfoot subjects in walking and jogging were significantly larger than those of the normal footed subjects. To better understand the relationship between this phenomenon and the injury of the foot, further research is needed.