In this study a dynamic model has been developed for two legs walking machinefor analysing the impact process. Energy dissipation in impact process due to legcontact with the surface was determined. Then the principle of maximum energydissipation was applied to find the vertical and horizontal reaction at the points ofcontact of legs with the surface. Also the relation between the angle of compass(legs angle see fig1), the angular velocity prior impact and the vertical andhorizontal reactions at the points of contact is studied in order to determine theoptimumangle that gives a stable walking process and reduces vibration.