A resistance determining system is used to determine an offsetting resistance of a mainboard to establish a predetermined offset voltage. The system includes an input equipment, a single-chip, and a resistor. The input equipment sets a predetermined standard voltage of the mainboard. The resistor supplies the mainboard with various resistances, to adjust real output voltage of the mainboard. The single-chip monitors the real output voltage of the mainboard, and adjusts the resistance of the resistor until the difference between the real output voltage and the predetermined standard voltage is equal to a predetermined offset voltage.