This paper presents a complete dynamic model of a five-link biped walking on the horizontal ground. The single support phase (SSP), double support phase (DSP) and double impact are included in the model. The dynamic equations for SSP and DSP are derived using Lagrangian formulations and the equations for the double impact are derived using both Newtonian method and integration method. The detailed forms of the biped dynamic model are provided in this paper. The model is tested by computer simulations. This work forms a basis for studies of motion planning and control of a full range of bipedal working.