A charger with output voltage compensation includes an AC/DC circuit, an output interface, a charge controller, and a compensator. The AC/DC circuit converts an AC source into a DC source. The voltage of the DC source is elevated by a transformer and used to charge a battery via the output interface. The charge controller can further increase the voltage of the output interface in order to accelerate battery charging. The compensator compensates the voltage wasted by the circuit. Therefore, the invention can overcome energy waste caused by the internal resistance in the circuit, achieving satisfactory charging effects.