Abstract: The development of studies on passive dynamic walking is reviewedbriefly. The nondimensional equations of a planar straight leg passivedynamic biped model with round feet are derived and a simulationis carried out. The basin of attraction of the model obtained by the cell mapping method is larger than that of pointfeet models, proposed by other researchers. These results indicate thatthe model can walk stably from an initial state far from the fixed point.