An approach using physical-sensor difference and virtual-sensor difference based algorithm to visually and quantitatively confirm lower limb posture was proposed and a wearable sensor system was developed. Flexion/extension (FE) and abduction/adduction (AA) hip joint angles and FE knee joint angle were estimated for orientations of the lower limb segments; the knee and ankle joint trajectories were calculated using the segmental orientation angles and lengths to estimate the positions of lower limb joints. In the wearable sensor system, an accelerometer on the hip and two MAG³s (inertial sensor module) on the thigh were in grouped to measure the data for estimating the thigh orientation and knee joint position using the double-sensor difference based algorithm. Two MAG³s on the thigh and shank near the knee joint were in grouped to measure the data for estimating the knee joint angle and ankle joint position using the virtual-sensor difference based algorithm. Compared with the camera motion capture system, the correlation coefficients in five trials were above 0.89 for the hip FE angle, higher than 0.9 for the hip AA angle and better than 0.88 for the knee FE angle. There was no integration of acceleration or angular velocity for the joint rotations and positions in this method. The developed wearable sensor system was able to visually and quantitatively confirm the lower limb posture with fewer sensors and higher accuracy than with other methods. It can be used as a substitute for the camera system for patient gait posture analysis in daily life.