Abstract: We have introduced the idea of passive-dynamic walking(PDW)for the purpose of controlling knee orthosis with powered joint smoothly. We have designed a passive walking machines without and with knee joint but without any actuators, and examined by simulation experiments as well as real experiments. Next, we have developed a walking machine using pneumatic actuators. In order to walk on the flat surface with little energy consumption, we have adopted quasi-passive-dynamic walking(QPDW). We have examined walking conditions of the walking machine.