Objective To study the effect of lateral wedge insoles with different stiffness on the knee adduction moment (KAM) under walking condition. Methods The gait of 15 healthy males wearing lateral wedge insoles with different stiffness was tested. The kinetics and kinematics data were collected by optical motion capture system and ground reaction force platform. The KAM and its peak values were calculated by Visual 3D software. The differences in peak KAM under 3 walking conditions (shoes only, shoes with softer or harder lateral wedge insoles) were analyzed. Results Compared with walking with the softer lateral wedge insoles, walking with the harder lateral wedge insoles could reduce the 1st and 2nd peak KAM by 9.3% and 9.7%, respectively, with significant statistic differences. Conclusions Increasing some stiffness of lateral wedge insoles can further reduce the pressure and wear on medial compartment of the knee joint, which may relieve the symptom of knee osteoarthritis.