This paper investigates three-dimensional quasi-passive dynamic bipedal walking on level ground with flat feet and compliant ankles. We present pointmass dynamic models to analyze and tune the characteristics of the bipedal gait. Simulations are carried out to study the effects of flat feet and ankle stiffness on the stability in both sagittal plane and frontal plane. Based on the theoretical analysis, we construct a robot prototype driven by hip actuation. The implementing of flat feet and coordination of ankle complex allow the prototype to walk on natural ground.