This paper numerically studies the inrush current phenomenon in a DC-DC converter with isolating transformer. It uses Jiles-Atherthon method to model magnetic core used in isolating transformer. Due to fast change of switching duty-cycle to suit the load change, magnetic core experiences a large change of flux density, thereby inducing a large inrush current, though the operating point of the magnetic core is still well below the saturation point. This paper also shows that by introducing an intermediate duty-cycle stage, such kind of inrush current can be significantly reduced.