A new control system of the bidirectional isolated dc-dc converter which can obtain the output voltage much higher than the turn ratio of the transformer, will be presented. The output terminals of two single phase bridge inverter A and B are connected through an isolated transformer, and the pulse width of the inverter of higher d.c. voltage is narrowed to make equal the fundamental a.c. voltage of both inverters. After doing so, to control the power flow, the phase angle between both inverters is controlled. From the experimental results of the charge-and-discharge using Li-Po, the effectiveness of this system has been checked.