An output buffer includes a first output transistor, a first switch, a second switch and a third switch. The first output transistor is connected to a first operational voltage for outputting the first operational voltage as the data signal. The first switch is connected to a bulk of the first output transistor for receiving an enable signal. The second switch is connected to the first switch and a second operational voltage for receiving the enable signal, wherein the second operational voltage is lower than the first operational voltage. The third switch includes a first terminal connected to the bulk of the first output transistor, a control terminal connected to the first switch, and a second terminal connected to the first operational voltage.