A display controller and a related method for calibrating display driving voltages according to input resistance of a monitor. The display controller has a digital-to-analog converter (DAC) for converting a display data into a corresponding display driving voltage. The DAC has a current mirror circuit for converting the display data to an output current in proportion to a reference current according to a mirror ratio, and a voltage calibration circuit for adjusting the mirror ratio according to the display driving voltage and a predetermined display driving voltage to make the display driving voltage approach the predetermined display driving voltage with an adjustment of the output current.