An Ankle-Foot Orthosis (AFO) is a rehabilitation device that supports and aligns the ankle and the foot to improve the functions of the ankle and foot (Wu, 1990). In today’s orthotic industry, polypropylene (PP, a Colyene™ Co-polymer plus Fleshtones and Colors) is used as a major orthotic plastic due to its high weight-strength ratio, high fatigue resistance, light weight, and excellent molding characteristics (1990). However, earlier failure and improper geometry design bring inconvenience in mobility and discomfort to many patients. A literature search shows several investigations have been conducted. A 3-D Finite Element Model (FEM) was developed by Chu, et al. (1995). The analysis includes the computation of the stress level and the determination of locations of stress concentration. Although the 3-D model provided useful information, limited dynamic results were obtained. Yamamoto, et al. (1993) made a comparative study on the mechanical characteristics of plastic AFOs. Eleven AFOs were tested with the motion of dorsiflexion/plantarflexion and inversion/eversion using a muscle-training machine. The objective of this experimental study is to understand the performance of five AFOs used during walking and the resulting deformation developed while under loads. It is intended that the design parameters of AFOs can be evaluated and modified.