Several studies suggested portable systems that measure horizontal and vertical ground reaction force (GRF) during walking. However, systems that could measure horizontal GRF usually have larger sensing modules than systems that only measure vertical GRF. Using larger sensing modules, the system weight and height, and altered shoe stiffness may affect gait behaviors. We suggest methodology for estimating horizontal GRF from measured vertical GRF data. Compliant leg model simulation was employed to relate vertical and horizontal component of GRF during walking. Initial conditions and model parameters are selected to minimize the difference between simulated and measured vertical GRF. Our method predicted horizontal GRF with average root mean square error less than 10% and correlation coefficient higher than 0.95 compared with measured GRF data. The proposed method might be applied to develop simple GRF sensing systems with less effect on natural walking.