Biped robots based on limit cycle walking paradigm commonly need less energy than other bipeds and generate pretty natural gait. However, this paradigm is not much practical to apply real biped yet. To increase performance and to realize a practical biped, the model needs further studies on its stability, disturbance rejection capability, and versatility, the ability to perform various gaits, etc. We analyzed stability of the flat foot model. One is flat foot without heel and another is flat foot with heel. Flat foot model with heel have stable one–gait cycle on steeper slopes and also have larger basin of attraction than the model without heel.