Abstract: Passive dynamic walking is one of the important topics in the field of biped robots, but the effect of friction at the hip joint upon the gait of a passive dynamic walker is still a controversial problem. Thusly the study of the modeling, simulation and gait of a straight leg biped passive dynamic walker with Coulomb friction at the hip joint is presented. The dynamic equations of a passive dynamic walker with Coulomb friction at the hip joint are first derived, and these equations are non-linear and non-smooth implicit differential equations. Then, a new numerical scheme for the dynamic equations is proposed by using a trial-and-error method. Finally, the effect of Coulomb friction on the walker's gait is investigated by numerical simulation. The results of the present study are expected to be useful for the modeling and simulation of passive dynamic walking.