A computer system including a power supply, a plurality of mainboards, and a power controller is provided. Each of the mainboards corresponds to a standby voltage, respectively. The power supply generates a standby power, and generates a main power according to a power enabling signal. The power controller is coupled between the power supply and the mainboards, for generating the power enabling signal and a control signal according to whether an amount of the mainboards is greater than a predetermined value, and selectively outputting the control signal to at least one of the mainboards. When the mainboards receive the control signal, regardless being in a booting state or a non-booting state, the mainboards receive the main power and converts the main power into a standby voltage corresponding thereto. When failing to receive the control signal, the mainboards convert the standby power into the standby voltage corresponding thereto.