To date, most passive walkers have been designed with arc-shaped feet rigidly attached to the legs. We developed a simple 3D straight-legged passive walker with flat feet and ankle springs. The flat feet are connected to the legs with springs at the ankles that produce torsional force while the stance leg is on the ground, mimicking the motion of simple 3D passive walkers with arc-shaped feet. This helps to prevent slipping and to handle the disturbance behavior of the robot. Flat feet are particularly advantageous for yaw stability. Our 3D passive-walker robot with a 0.84-m leg can walk the full length of a 1.8-m slope at about 0.44 m/s.