Objective: The aim of this study was to investigate the periodicity of the lower extremity joint flexion/ extension angle to compare the local stability between young and elderly women during walking on a treadmill. Method: Eighteen young women (mean age=21.2±1.6 y; mean mass=57.1±6.1 kg; mean height=1.61±0.04 m) and 18 elderly women (mean age=66.4±1.2 y; mean mass=55.4±8.3 kg; mean height=1.56±0.04 m) participated in this study. Approximate entropy (ApEn) was used to determine the periodicity in the lower limb joint angles. Results: The ApEn values of the two groups were statistically greater in the surrogate data test than in the original time series data (p<.05). The periodicity of the hip and ankle flexion/extension angles decreased in the elderly women group compared with the young women group (p<.05). The periodicity of the lower extremity joint flexion/extension angle showed that the ankle joint increased dominatingly in both groups (p<.05); the hip joint decreased compared with the knee joint in the young women group; and the knee joint decreased compared with the hip joint in the elderly women group (p<.05). Conclusion: These results suggest that the lower extremity joint flexion/extension angles of the young and elderly women during walking contained random noises as well as biological signals. In addition, the differences in the periodicity in the lower extremity joint between the young and elderly women may provide some insight in predicting potential falls and be used as a characteristic indicator for determining local stability in elderly women during walking.