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A stationary force plate can only accurately measure complete ground reaction force (GRF) during no more than one stride, but the data of a successively measured multi-axial GRF in different environments is desired not only for researchers on gait analysis but also for clinical doctors. A wearable force plate system was developed by integrating small triaxial force sensors and 3D inertial sensors for estimating multi-axial GRF under free-living environments. In order to verify measures of the developed system, we adopted a combination system including a stationary force plate and an optical motion analysis system as a reference system that simultaneously measured triaxial GRF and the center of pressure (CoP) when a subject was required to wear the wearable system. The RMS difference and standard deviation of the two transverse components (x-axis and y-axis) and the vertical component (z-axis) of the GRF was 4.3Â±0.9N, 6.0Â±1.3N, and 12.1Â±1.1N respectively, corresponding to 5.1Â±1.1% and 6.5Â±1% of the maximum of each transverse component, and to 1.3Â±0.2% of the maximum vertical component of GRF. The RMS distance between the two systems' CoP traces was 3.2Â±0.8mm, corresponding to 1.2Â±0.3% of the length of the shoe. Based on the experimental results, we can conclude that the wearable system as an alternative device can be used to measure CoP and triaxial GRF with an acceptable accuracy in non-laboratory environments.