Abstract: Compared with traditional biped robots, passive dynamicwalker has simpler structure and higher energy efficiency. However, it hasvery weak tolerance for disturbances. Inspired by the biomechanics of humanwalking, we added a torsional spring at the hip of the passive dynamicwalker to improve its stability. We studied the effect of spring stiffnesson the stability of passive dynamic walking by simulations and experiments.In the simulations, the basin of attraction used as a measure for stability,was obtained with the cell mapping method. The effect of spring stiffness onthe basin of attraction was then studied. The size of the basin ofattraction increases with the increasing spring stiffness. In theexperiments, we quantified the stability of each walker by observing passivewalking trials down a gentle slope of finite length for 100 times andrecording the fraction of trials which successfully walked to the end. Theexperimental results show that spring stiffness with a moderate value caninduce maximal stability for passive dynamic walker. The hip momentcontributed to the spring is quite similar to that of human in a walkingcycle. This might indicate that the spring improves the stability. Thespring torque in a whole walking cycle has positive effect on stability,which can be used to explain the simulation results. The discrepancy betweensimulations and experiments might be explained by the fact that the steplength decreases with increasing spring stiffness, to induce the increasingchance of foot scuffing at mid-stance in experiments, which was ignored insimulations.