A DC/DC converter, a power converter and a control method thereof are disclosed, where the DC/DC converter includes an output circuit, a rectangular wave generator, a resonant tank, a detection unit and a control unit. The output circuit has a load. The rectangular wave generator converts an input voltage into at least one rectangular wave. The resonant tank provides a first voltage based on the rectangular wave for the output circuit. The detection unit detects a signal related to a state of the load. When the state of the load is light-load or a no-load, the control unit controls a working frequency or a duty ratio of the rectangular wave, so that the duty ratio of the rectangular wave is within a predetermined range, in which a voltage gain of the DC/DC converter is greater than another voltage gain under the condition of 50% duty ratio.