A shift register including shift register units substantially cascaded is disclosed. Each shift register unit is controlled by first and second clock signals opposite to each other for generating an output signal. Each shift register unit includes first and second switch devices and first and second devices. The first switch device provides the output signal through an output node. The first driving device drives the first switch device according to a first input signal to activate the output signal. The second driving device provides a first voltage signal, according to the first clock signal, to drive the first switch device and de-activate the output signal. When the first switch device de-activates the output signal, the second switch device provides the second voltage signal to the output node according to the second clock signal. A level of the first voltage signal is lower than a level of the second voltage signal.