A liquid crystal display device and a driving method thereof, which are capable of preventing generation of a residual image upon power-off, are disclosed. The liquid crystal display device includes a power supply unit for outputting a plurality of drive voltages after delaying the drive voltages, a voltage detector for monitoring one of the drive voltages, and outputting a power-off detect signal based on the result of the monitoring, a timing controller for increasing a frequency of a control signal in response to the power-off detect signal, and outputting the frequency-increased control signal, a gate driver for outputting a scan signal in response to the frequency-increased control signal, a data driver for outputting a constant voltage in response to another control signal from the timing controller, and a liquid crystal panel for applying the constant voltage to all sub-pixels of the liquid crystal panel in response to the scan signal.