Osteoarthritis (OA) of the knee is one of the most common diseases. For this chronic disease, modified sole structure can effectively prevent and relieve disease of the knee. The aim of this study is to explore the influence of unstable shoe with different stiffness on ground reaction force and external knee adduction moment. 17 healthy female volunteers were recruited, and every subject performed five walking trials in three shoes condition. The results showed that external knee adduction moment was decrease in all stance phase using soft unstable sole. The ground reaction force has no obvious changing between two shoes condition. The unstable shoes of this study mainly create instability in the medial-lateral direction and lead to external knee adduction moment increase.