This paper proposes a system and its control method to boost part of the battery voltage for electric vehicle. This system consists of the powertrain of electric vehicles and a bi-directional isolated DC-DC converter called dual active bridge (DAB). The powertrain of electric vehicles in the market has the problem that its performance degrades due to fluctuations in battery voltage. In order to solve this problem, a DC-DC converter is often inserted serially between the battery and the inverter. However, in this solution, the conduction loss of the DC-DC converter always occurs. In this paper, as a solution to the above problems, a voltage boosting system using DAB converter is shown.