In this paper, seamless model of bi-directional DC-DC converter which is applicable to bi-directional power transfer is proposed. Conventional modeling method of bi-directional DC-DC converter is complex, because different models are needed for different directions of power flow. On the other hand, seamless model can be applied for both directions of power flow. Seamless circuit model is expressed by voltage source and bi-polar current source, and polarity of current source shows direction of power flow. In this model, control loop need to be switched by direction of power flow. Therefore analyzing and controlling bi-directional DC-DC converter is much easier than conventional way. Seamless dynamic model are derived by state-space averaging method. Analytical, simulated, experimental results are investigated, and availability of seamless model is suggested.