A wearable sensor system for gait analysis is presented, which is composed of a combination of motion sensor modules and a 6-aixs reaction force sensor. Experimental study was implemented to estimate orientations of leg segments and measure foot reaction forces during walking. Based on the quantitative motion and force analysis, this system can detect the following gait phases from clinical study: initial contact, loading response, mid stance, terminal stance, pre swing, initial swing, mid swing and terminal swing.