AFO is a device, which is usually recommended for hemiplegics to help their pathologic gait. To date, no good engineering guidance is available. We analyze mechanical characteristics of AFO and evaluate structural stability effect of thickness of ankle foot orthosis (AFO). The study is to provide a guidance to make a functional AFO. Basic material characteristics such as Young’s modulus and Possion’s ratio were measured through material tests. Finite element models were made and analyzed based on various shapes and gait analysis. Young’s modulus of polypropylene is 1.15 GPa and Possion’s ratio is 0.35. Rigidity of AFO is more affected by the ankle width than the thickness of ankle area. The strain calculated by finite element analysis (FEA) in the neck area is twice as much as experimental results but the both of their trend show similar pattern. AFO was studied through its structural and functional analysis. Experimental structural analysis and FEA were made and compared.