In this paper, an isolated MHz resonant dc-dc converter with regulating capability is proposed. This proposed dc-dc converter works at fixed switching frequency and can be regulated by an auxiliary non-isolated PWM dc-dc converter. In the proposed converter, only a portion of power is transferred through two-stage, namely the DC transformer (DCX) stage and the auxiliary PWM dc-dc stage; and most of the input power is delivered to load directly through the DCX stage, which makes the total efficiency of the dc-dc converter higher than that of conventional two-stage converter. Finally, two 48 V/12 V-25 A prototypes are built to verify the theoretical analysis. The peak efficiency of them at 55 V input voltage are 96.7% and 97.4%, respectively. The power density of them reaches 370 W/in³.