A method of maximizing power efficiency for a power amplifier system comprises obtaining a power supply voltage; determining a first voltage level sufficient for a power amplifier of the power amplifier system to output an output power; determining a second voltage level lower than the first voltage level; determining whether the power amplifier is activated, to generate a determination result; determining to convert the power supply voltage into a supply voltage with the first voltage level or the second voltage level according to the determination result; and supplying the power amplifier with the supply voltage.