An exemplary voltage variance tester includes a first to a third testing circuits each comprising an adjustable power source and an electrical switch; a first to a third signal generators providing a first to a third signals respectively; a connector having a first to a third terminals connected to the first to the third testing circuits respectively for receiving the adjustable power sources, a fourth to a sixth terminals connected to the first to the third signal generators for receiving the first to the third signals, and a seventh terminal; and a control circuit connected to the seventh terminal of the connector for receiving a power on signal to turn on the first to third electrical switch, wherein, voltage variances of the motherboard are tested by adjusting the first to the third adjustable power sources.