This study examined the effect of anterior-posterior thigh marker placement on measuring knee joint angles using an optical motion capture system. The knee joint center displacement due to incorrect thigh marker placements may cause a significant error in the gait measurement. Therefore, the standard marker at the outermost on the thigh and the anterior thigh marker, at the same height as the standard one, were attached to three subjects, and the knee joint angles were obtained by the Plug-in gait model. Furthermore, a computer-aided design system was utilized to create the thigh and tibia segment models. By comparing the joint angles obtained from the segment models and the gait measurement, we have concluded that the knee joint angles during the stance phase might have been mainly affected by the knee joint center displacement. During the swing phase, the soft-tissue artifacts seem to have greatly affected the results.