A motherboard testing apparatus for automatically turning on or off a motherboard includes a pulse signal generating circuit for outputting a pulse signal, a first control circuit for outputting a first control signal to an I/O controller on the motherboard according to the pulse signal, and a second control circuit. The first control circuit outputs a low level first control signal when it receives a low level pulse signal, the I/O controller turns the motherboard on when it receives the low level first control signal. The second control circuit outputs a second control signal to the first control circuit which controls the motherboard to turn on again when the first control circuit receives the low level pulse signal a next time.